# Find a real entire function $f(z)$ asymptotic to $\ln(x^2+1)$ for real $x$.

Find a real entire function $f(z)$ asymptotic to $\ln(x^2 +1)$ for real $x$.

More specific I want $f(0)=0$ and $\frac{1}{2} \ln(x^2+1) < f(x) < 2 \ln(x^2+1)$.

Or prove it does not exist.

• What do you mean by real? That $f\vert_{\mathbb{R}}$ maps into $\mathbb{R}$? Also, typically by asymptotic one would mean that $\lim_{x \to +\infty} [f(x) - \ln(x^2 + 1)] = 0$, which is strictly stronger than your condition. Sep 1 '14 at 11:45
• @Travis : Not neccessarily stronger ; $f(x)$ could be close to $3 ln(x^2+1)$ for small $x$. That violates the condition.
– mick
Sep 1 '14 at 19:06
• I think it does exist. I will write a partial "answer", but its not a proof nor an example.
– mick
Sep 1 '14 at 20:47
• Since $\ln(x^2+1)$ is asymptotic to $2\ln x$, would it simplify the statement of the problem to simply use $\ln x$? That would make your question about whether one of the first examples of a non-entire function could have an entire function it is asymptotic to. Sep 1 '14 at 21:29
• Also, if you meet the asymptotic criterion you can add the appropriate multiple of $\exp(-x)$ to end up with $f(0)=0$. So that condition would be trivial. Sep 1 '14 at 21:32

This is an increased elaboration on my previous answer, which is at the character count limit anyway. It is equivalent to work with $f(x) \approx \exp(x)\ln(x+1)$ where $\text{entiref}(x) = f(x)\exp(-x) \approx \ln(x+1)$. In particular, the graphs have a more natural approximately $2\pi i$ exponential appearance with this approach, making it easier to understand where the zeros are.

$$f_{\ln(x+1)} = \sum_{n=0}^{\infty} a_n x^n \approx \exp(x)\ln(x+1)$$ $$f_{\ln(x^2+1)} = \sum_{n=0}^{\infty} a_n x^{2n} \approx \exp(x^2)\ln(x^2+1)$$

So from this point forward, I work only with the ln(x+1) equation, for theoretical simplicity. The Op's requirement forces $a_0=0$, and ideally $a_1=1$, since $\ln(x+1)=x-x^2/2...$

BEGIN solution2. Here is a different, apparently numerically identical result that gives the same numerical results as the answer I posted below. Here $\gamma \approx 0.5772156649015\;\;$ where $\gamma=$ is the euler-mascheroni constant.

$F(x) = C + \int \frac{1-\exp(-x)}{x}\;\;C=\gamma\;\;\;$ This is an asymptotic ln(x) function $F(x)$ is entire since (1-exp(-x))/x is trivially entire function, we integrate termwise, which leaves a constant C. F(z) is from https://mathoverflow.net/questions/26243/asymptotic-approximation-of-x-alpha-by-entire-functions on on mathoverflow; my observation is that numerically, $C=\gamma$.

Now, consider $F_1(x)=F(x+1)+\exp(-x)\cdot(b_0 + b_1x)\;\;\;$ For any constants b0 and b1, $F_1(x)$ is an asymptotic solution to $\ln(x+1)$ as x gets arbitrarily large. We choose $b_0+b_1 x$ such that $F_1(0)=0$ and $F_1'(x)=1$.

$b_0 = -F(x+1)\approx -0.2193839343955 \;\;\;$ asymptotic ln(x) at x=1

$b_1 = 1 - F(x+1) - \frac{d}{dx}F(x+1)\approx 0.1484955067759\;\;\;$ derivative of asymptotic ln(x)

$F_1(x) = F(x+1)+\exp(-x)\cdot(b_0 + b_1\cdot x)\;\;\;$ asymptotic ln(x+1) function

What is perhaps surprising, is that $F_1(x)$ is identical to the solution posted below to the limits of numerical accuracy, so that using the solution below setting $a_0=0, a_1=1$, $F_1(x)=\exp(-x)f(x)\;\;$ So all of the plots and Taylor series are good. END solution2

Then we define all of the other Taylor series coefficients of f(x) as the following limit which is the Cauchy integral of of $\;(\exp(x)\ln(x+1))\;$, while ignoring the fact that there is a logarithmic branch cut at the negative real axis. However, that branch discontinuity is multiplied by exp(-x) at the negative real axis, so as x gets bigger, the discontinuity becomes arbitrarily insignificant, so the equation for all of the Taylor series coefficients of f(x) converges in the limit as r goes to infinity.

$$f(x)=\sum_{n=0}^{\infty} a_n x^n\;\;\; \exp(-x)f(x)\approx \ln(x+1)$$ By definition, for any entire function $f(x)$, we have for any value of r $$a_n = \oint x^{-n} f(x) = \int_{-\pi}^{\pi} \frac{1}{2\pi} (re^{-ix})^{-n} f(re^{ix}) )\; \mathrm{d}x\;\;$$ The conjecture is that this is an equivalent definition for $a_n$, where $f(x) \mapsto \exp(x)\ln(x+1)$ $$a_n =\lim_{r\to\infty} \int_{-\pi}^{\pi} \frac{1}{2\pi} (re^{-ix})^{-n}(\exp(re^{ix})\ln(1+re^{ix}))\; \mathrm{d}x\;\;$$

This is the equation I used to generate the f(x) Taylor series, where asymptotically $\ln(x+1)\approx f(x)\exp(-x)$. A pretty good approximation for the Taylor series coefficients is $a_n \approx \frac{\ln(n+1.5)}{n!}$, for larger values of n. Interestingly, the equation also has negative Taylor series coefficients. $a_{-1} \approx -0.3678794411714=\frac{-1}{e}$, which needs investigation. Because of this for large values of x, $f(x)\approx \frac{1}{e\cdot x} + \exp(x)\ln(x)$; this also allows approximating the location of the zeros of f(x); see below. For large negative values of x, the exp(-x) term cancels out the ln(x), leaving just $\frac{1}{e\cdot x}$

If we use the equation above for a0 and a1, we get $a_0 \approx 0.2193839343955$ instead of $a_0=0$, and we get $a_1 \approx 0.8515044932241$ instead of $a_1=1$. For large enough values x, the limit equations converge very slightly faster than using $a_0=0$ and $a_1=1$, but for both sets of equations, the error term for f(x) as x gets larger is approximately $f(x)\exp(-x) - \ln(x+1) \approx \exp(-x)\;\;$

Here is complex plane graph of $f(x)\exp(-x)$ from -40 real to +100 real, and from -10 imaginary to +100 imaginary, with grids every 10 units. You can see the zeros pretty clearly, near the imaginary axis, occurring approximately every $2\pi i$. To the right of the zeros, as $\Re(x)$ increases, the function approximates $\ln(x+1)$ increasingly well, with the error term decreasing exponentially.

Here is the nearly identical graph using $a_0=0$ and $a_1=1$.

update I added the zeros of the asymptotic approximation. The zeros of f(x) are interesting, both in and of themselves, and because I conjecture they can be approximated increasingly well by the zeros of $\;\;\exp(z)\ln(z+1) - a_{-1}/x^1 - a_{-2}/x^2 ...\;\;$ where the $a_{-1}, a_{-2}, a_{-3}$ terms are the Laurent series terms from the equation for the Taylor series approximation terms of $\exp(x)\ln(x+1)$. Since $\exp(x)\ln(x+1)$ has a branch, the Laurent series appears to be a non-convergent series, with some ideal number of terms for best convergence at a particular radius. $a_{-1}=-e^{-1}\; a_{-2}=2e^{-1}$, although I don't have a closed form equation for the general Laurent terms. Here are the first 18 zeros of f(x), from the first plot above. An eight term Laurent series expansions gives the zero approximation to 10 decimal digits accuracy for the 12th ... 18th zero below; a one term Laurent series is accurate to about 0.03 for the same zeros, using the zeros of the approximation $\;\;\exp(z)\ln(z+1) - a_{-1}/x^1 - a_{-2}/x^2 ...$

zero[ 1]= -0.3236449221346
zero[ 2]= -4.014193883364 + 6.790134224598*I
zero[ 3]= -4.782847137525 + 13.31987455364*I
zero[ 4]= -5.248507475714 + 19.72207503441*I
zero[ 5]= -5.583409659397 + 26.07792021174*I
zero[ 6]= -5.844835249373 + 32.41104823228*I
zero[ 7]= -6.059115679636 + 38.73113851434*I
zero[ 8]= -6.240572105211 + 45.04296385204*I
zero[ 9]= -6.397864071944 + 51.34917648054*I
zero[ 10] -6.536627830306 + 57.65137956884*I
zero[ 11] -6.660737008441 + 63.95060483952*I
zero[ 12] -6.772967940649 + 70.24754933585*I
zero[ 13] -6.875378096927 + 76.54270265230*I
zero[ 14] -6.969534334368 + 82.83641978994*I
zero[ 15] -7.056657240617 + 89.12896507913*I
zero[ 16] -7.137716050140 + 95.42053981217*I
zero[ 17] -7.213493140112 + 101.7113002628*I
zero[ 18] -7.284629098684 + 108.0013698029*I

Here is the Taylor series for $f(x)\approx\exp(x)\ln(x+1)$, using the very slightly faster converging a0, a1 terms mentioned above. Using these terms, the error term at x=100 is $1.3\cdot 10^{-46}$, where as with $a_0=0$ and $a_1=1$, the error term at x=100 is $5.4\cdot 10^{-43}$. Both equations converge to $\ln(x+1)$ extremely well.

{f=     0.21938393439552027367716377546012164903104729340691
+x^ 1*  0.85150449322407795208164000529866078158523616237514
+x^ 2*  0.60969196719776013683858188773006082451552364670345
+x^ 3*  0.24727084200877260508101937252286665235657083855723
+x^ 4*  0.070454237461720398335802452337748546617262159063916
+x^ 5*  0.015514959258055615348055736435257535834290205606430
+x^ 6*  0.0027879556271385450461521399883419561022391116619218
+x^ 7*  0.00042343857303596842945484648320523488742621945510136
+x^ 8*  0.000055717110916929110265184223440405648797525953715792
+x^ 9*  0.0000064689315465613287984287229359980719367599295991558
+x^10*  0.00000067213904668370689430101718748196623687022488449444
+x^11*  0.000000063204847149802332385921949172748610132134439176877
+x^12*  0.0000000054285569642604407680708170116601545269943834259260
+x^13*  0.00000000042910740320956106423569008281592719741927414285725
+x^14*  3.1418468865280295945461413540755545520739344048138 E-11
+x^15*  2.1425369039097961105695603176524807543930213727397 E-12
+x^16*  1.3672928648282075281735514326247637522828467835981 E-13
+x^17*  8.1995517641975688990361908015594450732188572847666 E-15
+x^18*  4.6377308388917617755416866780753189294535858548614 E-16
+x^19*  2.4821126724601902380920373280630327756997651176156 E-17
+x^20*  1.2606719046483132207278964996529137034310801653575 E-18
+x^21*  6.0923442791198460616101849955646734727055475426474 E-20
+x^22*  2.8079995403516031653048670415220199656935513917287 E-21
+x^23*  1.2370137317651528145106660121454618616618021777442 E-22
+x^24*  5.2187930072739089066071222548846663862241677980797 E-24
+x^25*  2.1123486321245979792606010628000124337396338084787 E-25
+x^26*  8.2163766545660957824472398598724391270013731475031 E-27
+x^27*  3.0759418931600743796126123440882303023524843343588 E-28
+x^28*  1.1098736820994923462096604771706409816554669646912 E-29
+x^29*  3.8648911676601180671741197586420955572689994468279 E-31
+x^30*  1.3004706013702334777488569775930847537529207016767 E-32
+x^31*  4.2331064507935496174504431896326073557096048452120 E-34
+x^32*  1.3343723921157428753132427362570458917902218486584 E-35
+x^33*  4.0774528116149739365250732170802794193580802691886 E-37
+x^34*  1.2089361182271519384060643613133734914839471552756 E-38
+x^35*  3.4810056234452616644964337736964845589560188290895 E-40
+x^36*  9.7421664484430176303796373993032267570284634196311 E-42
+x^37*  2.6521505176505907494723470420008066610346923816951 E-43
+x^38*  7.0283996498005696166583025896724096774216357574486 E-45
+x^39*  1.8144174173021233476867258355660330358668184124351 E-46
+x^40*  4.5659540404409082356421503479768604519187067193552 E-48
+x^41*  1.1207686810110486409883560903525616794092295495464 E-49
+x^42*  2.6850577301937551618242191836068060564918068132606 E-51
+x^43*  6.2819577588140613841462549531955229419014474334054 E-53
+x^44*  1.4360813674834393720403422636778389739251052369530 E-54
+x^45*  3.2094734988354378288387663062978408954466414938866 E-56
+x^46*  7.0157997484846807249151770281709128478370705850365 E-58
+x^47*  1.5007822560201339126602951659771078434157734104338 E-59
+x^48*  3.1430761652431388655439842011443189600666082127433 E-61
+x^49*  6.4473335564509965548673646210563187263660093241493 E-63
+x^50*  1.2959161045965261908561467302925777782072867938029 E-64
+x^51*  2.5534144724382313792659205799484989833173576390613 E-66
+x^52*  4.9338130803591680583924265678039252034985623629524 E-68
+x^53*  9.3524152384202180018364853504610420416515905921669 E-70
+x^54*  1.7398075538196532343047190893767842230161523880674 E-71
+x^55*  3.1773555973291832197507306637969448627194531667074 E-73
+x^56*  5.6985317083593455545811990207835484446639356496487 E-75
+x^57*  1.0039979522112221715454278482114685560423544754674 E-76
+x^58*  1.7382436736570566722715043961789314008279939597400 E-78
+x^59*  2.9581968091895712886859196387499679815774690077017 E-80
+x^60*  4.9500345481668768091645414814908578287049973279667 E-82
+x^61*  8.1465951979683931116449021822984407224263385834921 E-84
+x^62*  1.3190120799010100854185574730262452899864761940633 E-85
+x^63*  2.1015528961291671603995233296366455172924808494176 E-87
+x^64*  3.2958038933467894377441053430895870647560098854354 E-89
+x^65*  5.0888423898400211028800293697067102179963830918628 E-91
+x^66*  7.7377922413134448213425367419879000759190973943799 E-93
+x^67*  1.1589274245689485179747923499196568650172183310758 E-94
+x^68*  1.7101500152755765037730124191662035791668050237802 E-96
+x^69*  2.4868282044688399528218459019092125322039065496681 E-98
+x^70*  3.5643721076378503037194989013977327146522781270671 E-100
+x^71*  5.0365761882909228472139735217683990141049549854801 E-102
+x^72*  7.0176196362338359810889929055701469677749180045100 E-104
+x^73*  9.6434138267516752008164145939953983291855551829716 E-106
+x^74*  1.3071955105591949087565453199703381242882948848579 E-107
+x^75*  1.7482319020580060945603412047058210203235570469467 E-109
+x^76*  2.3071941360846680888173341368637863861356891384686 E-111
+x^77*  3.0051880372175153679251142372180353100357337811609 E-113
+x^78*  3.8639848980310148959795556666198626468621686488783 E-115
+x^79*  4.9050947220902219166928252644812421712832374982310 E-117
+x^80*  6.1486209277509300441055370767422161831734083726865 E-119
+x^81*  7.6119296226783001347612832366291798557314465694929 E-121
+x^82*  9.3081898162333480340693940360270298271097919676064 E-123
+x^83*  1.1244863581962953843405712454205644508890910834887 E-124
+x^84*  1.3422244663037402781248207041407667441373049783035 E-126
+x^85*  1.5832157692575953817972496895341557987585900245206 E-128
+x^86*  1.8456935680923038373690672461801317698216166569041 E-130
+x^87*  2.1268788885714486521103434144307569143493972779958 E-132
+x^88*  2.4229662699295898957611048751470881887684611124016 E-134
+x^89*  2.7291655949059223941427637152794841530085849008071 E-136
+x^90*  3.0398035416946135636028990121365440959484383440743 E-138
+x^91*  3.3484840025418832038484151939256019042316047042909 E-140
+x^92*  3.6483022558183007101795318927317531607056221358767 E-142
+x^93*  3.9321032157621411141379422190462544049587528318256 E-144
+x^94*  4.1927701514859389140909389981673311084588771617862 E-146
+x^95*  4.4235272669923616137958106471637278748511939312572 E-148
+x^96*  4.6182377898763770355728498617515900781695607529365 E-150
+x^97*  4.7716789320345269781101284578089526651825262191294 E-152
+x^98*  4.8797763202677976199850319301888485449889336685429 E-154
+x^99*  4.9397831542631726529459190348494517789065467748272 E-156
+x^100*  4.9503931971143089329866743631577716880633235705229 E-158
+x^101*  4.9117813862635127248462260315296755390071135150903 E-160
+x^102*  4.8255709416189904342543469753027592474167698000280 E-162
+x^103*  4.6947308858382379843202270819872899643956342976271 E-164
+x^104*  4.5234124441475715033896158230559723452500482772060 E-166
+x^105*  4.3167364886479819276205447338170893220682821083861 E-168
+x^106*  4.0805467669288668471298023671114476107474410370126 E-170
+x^107*  3.8211449612773525249192372118017444018131856111826 E-172
+x^108*  3.5450236453885623527062371927933348285893783244252 E-174
+x^109*  3.2586120424601915408821719193404432011260503836298 E-176
+x^110*  2.9680473428374280896879078486321870280699755753488 E-178
+x^111*  2.6789814810639341414957466989183587648059400797232 E-180
+x^112*  2.3964300066679809069756609257598126680493066023475 E-182
+x^113*  2.1246663171265192034320134663780201297776285986656 E-184
+x^114*  1.8671613336865434920788141519694894305563699913681 E-186
+x^115*  1.6265659181520794885171318855735103214157761752862 E-188
+x^116*  1.4047311124567213777863504355051806005317722540700 E-190
+x^117*  1.2027597222569246629766702724383644677792206755081 E-192
+x^118*  1.0210818811257405412205760707438273519479961799225 E-194
+x^119*  8.5954698330499079592176872001434088628307616728829 E-197
+x^120*  7.1752467396949083627525669755611112367810654016094 E-199
+x^121*  5.9400831951367278977066109854343018440263342670341 E-201
+x^122*  4.8771541485230735320907995716303265296217707544636 E-203
+x^123*  3.9718058841925562252766251048314798344922790935907 E-205
+x^124*  3.2083811838309789218291913075664470611019799938317 E-207
+x^125*  2.5709207542651528481611339463608225286555149097497 E-209
+x^126*  2.0437328270221585277476758986377294750969598331193 E-211
+x^127*  1.6118318268018194488610632610936588329782050541822 E-213
+x^128*  1.2612539851622949493154153063528759372492215918155 E-215
+x^129*  9.7926270498507619328506410212243514245012240937081 E-218
+x^130*  7.5445948936227155202620182841491189646789967759740 E-220
+x^131*  5.7681758076228812615679592722215704929589905103341 E-222
+x^132*  4.3765542601429201889262169463408459179255991447259 E-224
+x^133*  3.2956602838905411738805382291241815740049116399927 E-226
+x^134*  2.4631649944977078860504830835597112543953361790550 E-228
+x^135*  1.8272997644208747788789532851884909979544048724176 E-230
+x^136*  1.3455977312437524650106651013022331670727326497270 E-232
+x^137*  9.8363374946541029880070340468144678017437717958686 E-235
+x^138*  7.1381809615231028869961088518181511429796578831647 E-237
+x^139*  5.1428106729307704472070264913462642532784020585123 E-239
+x^140*  3.6787047629802064484011221342722680042149497246155 E-241
+x^141*  2.6127207267821731473382386378136624615510117296017 E-243
+x^142*  1.8425387570385804039507331118314404220172451086195 E-245
+x^143*  1.2902904340009841445214910884150782226248437371062 E-247
+x^144*  8.9727763846206630162980372068308647530555321244133 E-250
+x^145*  6.1966326997131589119294793979444779621708671954377 E-252
+x^146*  4.2500588021933414188786367446496555498442143367240 E-254
+x^147*  2.8951085088163651685307592608650409676407846811886 E-256
+x^148*  1.9587799296431179662082428708715490369510075309207 E-258
+x^149*  1.3163677559370068573836516830365379999979844529815 E-260
+x^150*  8.7873767778245600528981456643467563036296478618569 E-263
+x^151*  5.8270809589890229808612834844831362636663636344119 E-265
+x^152*  3.8385902960181587756238432788550292473922758231242 E-267
+x^153*  2.5121191692848013794533790420901500081003370030882 E-269
+x^154*  1.6333343564851485478337357735030593621910826606898 E-271
+x^155*  1.0551026532424685686756552712554848084570219599032 E-273
+x^156*  6.7720043527468723160507297423038065311703945102949 E-276
+x^157*  4.3187746458416666065524473697031940949329731573402 E-278
+x^158*  2.7367954321478776213957675264172161034435658590798 E-280
+x^159*  1.7233760862141576032030570595919383469246108706966 E-282
+x^160*  1.0784275050796011077982183929187537940561651270387 E-284
+x^161*  6.7064398959285544549913919978797548147171073686236 E-287
+x^162*  4.1447669204107379871157734111016636130991567109028 E-289
+x^163*  2.5458439895971144620924646826388123030531039127643 E-291
+x^164*  1.5541876515984266195742476223477541830727134682877 E-293
+x^165*  9.4304260565984170069476612148378349099692912645524 E-296
+x^166*  5.6876304211685272334771942063474692914039348733514 E-298
+x^167*  3.4097255681881316920313209049478022239901230406786 E-300
+x^168*  2.0319410656792319220244794928747543943180951092304 E-302
+x^169*  1.2037099398982330855716119487558143790072006865748 E-304
+x^170*  7.0887048796743225068464281718891817078503406249907 E-307
+x^171*  4.1501264333709663750357784755238771806625142397642 E-309
+x^172*  2.4155722872164402402366408421263889644550305224395 E-311
+x^173*  1.3978409273259589099340096795576326774078783233527 E-313
+x^174*  8.0424620773383190885938430521414216620371780790823 E-316
+x^175*  4.6007457555223156980844993425732099710153763063800 E-318
+x^176*  2.6169149686347582147940021505008903429072959127999 E-320
+x^177*  1.4800868978782196236759264628558888802431943338838 E-322
+x^178*  8.3240550670500646398165455766546783041359923414080 E-325
+x^179*  4.6552879444777486212573378176444272280109059168592 E-327
+x^180*  2.5890212733452180092493549562050643632910049717503 E-329
+x^181*  1.4319095881582849186553262993046332768161728502099 E-331
+x^182*  7.8758924446023022551600836551281422742983316273664 E-334
+x^183*  4.3082540491380464458304335127991021808104560392678 E-336
+x^184*  2.3438681934420224431366294599560155852258577645381 E-338
+x^185*  1.2682599601734130141997140486447331276023793909678 E-340
+x^186*  6.8255761585313529030023214624502090447094176434611 E-343
+x^187*  3.6537504179850186529808586971817156694827461745866 E-345
+x^188*  1.9454470640383043345151522063744741073978647461371 E-347
+x^189*  1.0303701228040713220056802251251407296859058088742 E-349
+x^190*  5.4284092933935871432575826027630446684289531007288 E-352
+x^191*  2.8449161093678520514495836019835478787647186075990 E-354
+x^192*  1.4831867268736039429035720562759859189083222201095 E-356
+x^193*  7.6924289606812687736387887303841742969323513978913 E-359
+x^194*  3.9690278432912187579900658177233798314903108319170 E-361
+x^195*  2.0373674015999579024215378976211578308364897710997 E-363
+x^196*  1.0404724062589859131137824355131332521435210992424 E-365
+x^197*  5.2866324949365548094078654832688881232452728717961 E-368
+x^198*  2.6725524292132869175146706128752358666465067502235 E-370
+x^199*  1.3442591789564729820372553297261898601441735492047 E-372
+x^200*  6.7276043932161689622055463797647738572086166184884 E-375
+x^201*  3.3501898805653338209475704099873953414656205696260 E-377
+x^202*  1.6600482741408619782632712400525639393230694413890 E-379
+x^203*  8.1851190380409222601743302375742819631218670918510 E-382
+x^204*  4.0159919520697047756962795777863189382219028536132 E-384
+x^205*  1.9608062398059577710626370130100979535932257258769 E-386
+x^206*  9.5271038245275776436955979522559012420761763330331 E-389
+x^207*  4.6066131783811398794367271936266435768093148276505 E-391
+x^208*  2.2167023535320304489442025975165264592916611434582 E-393
+x^209*  1.0615681256049270688335788443041634417895975789096 E-395
+x^210*  5.0595649367762156018326402207801767212712516516386 E-398
+x^211*  2.4000106325239728954321198912425040707682272507762 E-400
+x^212*  1.1330722982063571446658985926135453313617945673750 E-402
+x^213*  5.3242228851643212061132930170268939117632059269563 E-405
+x^214*  2.4901102609759374945796280980499327458303881228791 E-407
+x^215*  1.1591888011476562915623901579261001784396161531425 E-409
+x^216*  5.3712138150299921355619109397593611599123741709992 E-412
+x^217*  2.4773233699268207877862835462388886546478705476704 E-414
+x^218*  1.1373501682539365954984102549761850117459697952691 E-416
+x^219*  5.1977584052351058915934485956926014724899321199762 E-419
+x^220*  2.3645987423128675227958881955755497702713404580007 E-421
+x^221*  1.0708466527742741848235811063637933736087185568317 E-423
+x^222*  4.8276356812403859567097281296510472005073074919968 E-426
+x^223*  2.1666457058982492763117755485891585568267652616207 E-428
+x^224*  9.6804661647641518073467320429143414790023337010315 E-431
+x^225*  4.3059429864857946197237910230468714805129111076487 E-433
+x^226*  1.9068323395955325335591466711305133567211579505800 E-435
+x^227*  8.4069312021948624849915116107499446884285747867687 E-438
+x^228*  3.6902150407655958254380893563093442792774280529903 E-440
+x^229*  1.6127365347504055955069556099905407950934412526019 E-442
+x^230*  7.0174777108830590245620859325682570613748759192320 E-445
+x^231*  3.0402741961576603620194757027175906767453325581009 E-447
+x^232*  1.3114952288328110327887496287280118389849605043720 E-449
+x^233*  5.6331460547306824417499990924856986023403966007448 E-452
+x^234*  2.4092044658830599569385868148282706810484794700070 E-454
+x^235*  1.0259887676107284313384139546012224728252003814666 E-456
+x^236*  4.3507660565262587322577700129128263635237342530769 E-459
+x^237*  1.8371763626014710809425480702452460554986982347759 E-461
+x^238*  7.7251283884274221825716125421795201716168565884387 E-464
+x^239*  3.2347296144298196095574184027015663208641347292147 E-466
+x^240*  1.3488240565723773570312053678447236861669572266031 E-468
+x^241*  5.6009954020913014783199103742920378160525136326225 E-471
+x^242*  2.3161955123320472025791616310545782156257726459461 E-473
+x^243*  9.5387778001852536493119022676929455071006216296259 E-476
+x^244*  3.9122368065366132896010583186772364320463011533886 E-478
+x^245*  1.5980108789305212015372959236632205470775444963553 E-480
+x^246*  6.5007546043738560791390486017894067129860361472177 E-483
+x^247*  2.6338100282375868123612084089514438680307662266472 E-485
+x^248*  1.0627934950764545348587435908647804252264441067403 E-487
+x^249*  4.2713402573825864197269787671449632181077221819674 E-490
+x^250*  1.7097684889342256728475583262169177605789830055375 E-492
+x^251*  6.8167170690893784857460923010713080991746238186594 E-495
+x^252*  2.7069794280609758907801550453849263081079974223609 E-497
+x^253*  1.0707133556414323052704577339192124817491334685693 E-499
+x^254*  4.2183912744938588321093927922482136936542473909198 E-502
+x^255*  1.6554368545070885644250347049640618354805092820812 E-504
+x^256*  6.4710862686884100831842289143927412177905065613502 E-507
+x^257*  2.5196905612142926143864290147382079645323080681317 E-509
+x^258*  9.7730307507476447488074147080763105480378621542793 E-512
+x^259*  3.7759818172532326284300013425827016757220246607830 E-514
+x^260*  1.4533010325546245227702971437250611589512219363578 E-516
+x^261*  5.5720212566576728141525208361883109280030709532657 E-519
+x^262*  2.1281774067436817198358112855254374840650280160364 E-521
+x^263*  8.0974283036088274826054118150946671135808445171292 E-524
+x^264*  3.0692827850287978024970428231289811214392137446404 E-526
+x^265*  1.1590000294229090885478486724562257863666147914118 E-528
+x^266*  4.3600647297857883309324682001259170855557193684051 E-531
+x^267*  1.6340732544631760993127019842572065195567965663052 E-533
+x^268*  6.1013410862961239302394223399514090175757477926474 E-536
+x^269*  2.2696575831980264118461130365373784092791445988453 E-538
+x^270*  8.4116781099102540484574194196715117268955325970347 E-541
+x^271*  3.1059766267867071243511144656800517649914940447924 E-543
+x^272*  1.1426490927215579200366990108987664265375732364114 E-545
+x^273*  4.1882501517153093051205893495243755206888706010413 E-548
+x^274*  1.5295483910899009974549524278253059947136740350680 E-550
+x^275*  5.5655808938547381233123139109469887436537848728330 E-553
+x^276*  2.0178096700795144737589310032739830097475781096231 E-555
+x^277*  7.2891691901754626842544881489312287505875259885583 E-558
+x^278*  2.6236727486199870277808066484473897349295341274922 E-560
+x^279*  9.4098072723088711513345530197835567174368153167740 E-563
+x^280*  3.3627672674347540093336157394946284797733972139982 E-565
+x^281*  1.1974667459520276815693244749893371561510457411718 E-567
+x^282*  4.2489946917606168696332967261626913487324367636916 E-570
+x^283*  1.5023477183994287175822262696270015833831770514219 E-572
+x^284*  5.2932415495228858569415743927490859755197170009214 E-575
+x^285*  1.8584262661552812916649197258771481519785514774630 E-577
+x^286*  6.5019957821473725533639805524051143831695631869323 E-580
+x^287*  2.2668932860841137891509077343544728652299429428693 E-582
+x^288*  7.8759653025974339712690776027586564551960357897210 E-585
+x^289*  2.7269054608787705338470784012530894649219706194811 E-587
+x^290*  9.4088197056789576557674817839914784306688713683354 E-590
+x^291*  3.2352225499222988825731449809832760704245792210012 E-592
+x^292*  1.1086188574907933487350934313471122722317816555556 E-594
+x^293*  3.7859471734049936555531848945534594819127400353898 E-597
+x^294*  1.2885049527100059791377750041493628364521500625936 E-599
+x^295*  4.3704073815830744250845512499365575137117715294106 E-602
+x^296*  1.4773623750012737580279726250722067623834873126979 E-604
+x^297*  4.9772149455130772543535991336437388993415469520252 E-607
+x^298*  1.6711865764110440014395419009948119834109551605523 E-609
+x^299*  5.5925201598622233712757741494808079484817947802005 E-612
+x^300*  1.8652589001443449088121380514937167555694076037155 E-614 }
• @AntonioVargas I also posted the first 18 zeros of the asymptotic approximation. Sep 9 '14 at 10:26
• I do not get the limiting integral formula. It seems to resemble fractional calculus, where you compute the t-th derivative with an integral. Also it seems different from the usual way ( like I tried in my answer ) to find an asymptotic with positive derivatives. Also , both a limit and an integral ? Can that not be simplified ?
– mick
Sep 9 '14 at 19:52
• its a cauchy integral, where if this were a normal analytic function, the radius does not matter. Here, instead, we take the cauchy integral as radius grows arbitrarily large. Sep 9 '14 at 20:02
• So its a contour integral. The one we use to get the Taylor coefficients for an entire function. Right ? But how does that give the fake function with the positive derivatives ? What makes things positive ? I still do not understand the integral ...
– mick
Sep 10 '14 at 7:50
• I didn't go through that set of details. Originally, I would have developed the function using a specific radius; same equation though. for our case, $f(x)=\exp(x)\ln(x+1)$. Now, take $h(x)=\log(f(exp(x)))$, which is still an increasing function, since f is increasing and always positive. In the fakefunction post, it turns out the optimal point for arithmetic is h'(x)=n; this is where the nth derivative is dominant. And if you generate r=exp(x), where h'(x)=n, then you get this very nice gaussian envelope for the nth derivative, cauchy integral. If it were entire, r doesn't matter. Sep 10 '14 at 8:30

The existence of such function is guaranteed by Carleman's theorem, for any two continuous real valued functions $f<g$, there exists a real entire function in between f(x) and g(x) for all reals. see: math overflow link

For numeric results, I used the Op's answer, and Tommy's suggestion, to generate the asymptotic function $f(x) \approx \ln(x^2+1)\exp(x^2$, and then the desired asymptotic function is $f(x)\exp(-x^2)$.

There were a lot of computational difficulties for generating this asymptotic result, both in generating $f(x)$ itself and even more so, in multiplying the Taylor series of f(x) by exp(-x^2). Here is the Taylor series for $f(x) \approx \ln(x^2+1)\exp(x^2)$, which gives results accurate to 35 decimal digits if x<=10, for $f(x)\exp(-x^2)-\ln(x^2+1)$. The asymptotic function's precision and error terms improve exponentially as x gets larger, for the infinite entire asymptotic Taylor series.

{f=
+x^ 2*  1
+x^ 4*  0.60969196719776013683857870360534506
+x^ 6*  0.24727084200877260508101935682305951
+x^ 8*  0.070454237461720398335802452260718126
+x^10*  0.015514959258055615348055736434881436
+x^12*  0.0027879556271385450461521399883401287
+x^14*  0.00042343857303596842945484648320522605
+x^16*  0.000055717110916929110265184223440405606
+x^18*  0.0000064689315465613287984287229359980717
+x^20*  0.00000067213904668370689430101718748196624
+x^22*  0.000000063204847149802332385921949172748610
+x^24*  0.0000000054285569642604407680708170116601545
+x^26*  0.00000000042910740320956106423569008281592720
+x^28*  3.1418468865280295945461413540755546 E-11
+x^30*  2.1425369039097961105695603176524808 E-12
+x^32*  1.3672928648282075281735514326247638 E-13
+x^34*  8.1995517641975688990361908015594451 E-15
+x^36*  4.6377308388917617755416866780753189 E-16
+x^38*  2.4821126724601902380920373280630328 E-17
+x^40*  1.2606719046483132207278964996529137 E-18
+x^42*  6.0923442791198460616101849955646735 E-20
+x^44*  2.8079995403516031653048670415220200 E-21
+x^46*  1.2370137317651528145106660121454619 E-22
+x^48*  5.2187930072739089066071222548846664 E-24
+x^50*  2.1123486321245979792606010628000124 E-25
+x^52*  8.2163766545660957824472398598724391 E-27
+x^54*  3.0759418931600743796126123440882303 E-28
+x^56*  1.1098736820994923462096604771706410 E-29
+x^58*  3.8648911676601180671741197586420956 E-31
+x^60*  1.3004706013702334777488569775930848 E-32
+x^62*  4.2331064507935496174504431896326074 E-34
+x^64*  1.3343723921157428753132427362570459 E-35
+x^66*  4.0774528116149739365250732170802794 E-37
+x^68*  1.2089361182271519384060643613133735 E-38
+x^70*  3.4810056234452616644964337736964846 E-40
+x^72*  9.7421664484430176303796373993032268 E-42
+x^74*  2.6521505176505907494723470420008067 E-43
+x^76*  7.0283996498005696166583025896724097 E-45
+x^78*  1.8144174173021233476867258355660330 E-46
+x^80*  4.5659540404409082356421503479768605 E-48
+x^82*  1.1207686810110486409883560903525617 E-49
+x^84*  2.6850577301937551618242191836068061 E-51
+x^86*  6.2819577588140613841462549531955229 E-53
+x^88*  1.4360813674834393720403422636778390 E-54
+x^90*  3.2094734988354378288387663062978409 E-56
+x^92*  7.0157997484846807249151770281709128 E-58
+x^94*  1.5007822560201339126602951659771078 E-59
+x^96*  3.1430761652431388655439842011443190 E-61
+x^98*  6.4473335564509965548673646210563187 E-63
+x^100*  1.2959161045965261908561467302925778 E-64
+x^102*  2.5534144724382313792659205799484990 E-66
+x^104*  4.9338130803591680583924265678039252 E-68
+x^106*  9.3524152384202180018364853504610420 E-70
+x^108*  1.7398075538196532343047190893767842 E-71
+x^110*  3.1773555973291832197507306637969449 E-73
+x^112*  5.6985317083593455545811990207835484 E-75
+x^114*  1.0039979522112221715454278482114686 E-76
+x^116*  1.7382436736570566722715043961789314 E-78
+x^118*  2.9581968091895712886859196387499680 E-80
+x^120*  4.9500345481668768091645414814908578 E-82
+x^122*  8.1465951979683931116449021822984407 E-84
+x^124*  1.3190120799010100854185574730262453 E-85
+x^126*  2.1015528961291671603995233296366455 E-87
+x^128*  3.2958038933467894377441053430895871 E-89
+x^130*  5.0888423898400211028800293697067102 E-91
+x^132*  7.7377922413134448213425367419879001 E-93
+x^134*  1.1589274245689485179747923499196569 E-94
+x^136*  1.7101500152755765037730124191662036 E-96
+x^138*  2.4868282044688399528218459019092125 E-98
+x^140*  3.5643721076378503037194989013977327 E-100
+x^142*  5.0365761882909228472139735217683990 E-102
+x^144*  7.0176196362338359810889929055701470 E-104
+x^146*  9.6434138267516752008164145939953983 E-106
+x^148*  1.3071955105591949087565453199703381 E-107
+x^150*  1.7482319020580060945603412047058210 E-109
+x^152*  2.3071941360846680888173341368637864 E-111
+x^154*  3.0051880372175153679251142372180353 E-113
+x^156*  3.8639848980310148959795556666198626 E-115
+x^158*  4.9050947220902219166928252644812422 E-117
+x^160*  6.1486209277509300441055370767422162 E-119
+x^162*  7.6119296226783001347612832366291799 E-121
+x^164*  9.3081898162333480340693940360270298 E-123
+x^166*  1.1244863581962953843405712454205645 E-124
+x^168*  1.3422244663037402781248207041407667 E-126
+x^170*  1.5832157692575953817972496895341558 E-128
+x^172*  1.8456935680923038373690672461801318 E-130
+x^174*  2.1268788885714486521103434144307569 E-132
+x^176*  2.4229662699295898957611048751470882 E-134
+x^178*  2.7291655949059223941427637152794842 E-136
+x^180*  3.0398035416946135636028990121365441 E-138
+x^182*  3.3484840025418832038484151939256019 E-140
+x^184*  3.6483022558183007101795318927317532 E-142
+x^186*  3.9321032157621411141379422190462544 E-144
+x^188*  4.1927701514859389140909389981673311 E-146
+x^190*  4.4235272669923616137958106471637279 E-148
+x^192*  4.6182377898763770355728498617515901 E-150
+x^194*  4.7716789320345269781101284578089527 E-152
+x^196*  4.8797763202677976199850319301888485 E-154
+x^198*  4.9397831542631726529459190348494518 E-156
+x^200*  4.9503931971143089329866743631577717 E-158
+x^202*  4.9117813862635127248462260315296755 E-160
+x^204*  4.8255709416189904342543469753027592 E-162
+x^206*  4.6947308858382379843202270819872900 E-164
+x^208*  4.5234124441475715033896158230559723 E-166
+x^210*  4.3167364886479819276205447338170893 E-168
+x^212*  4.0805467669288668471298023671114476 E-170
+x^214*  3.8211449612773525249192372118017444 E-172
+x^216*  3.5450236453885623527062371927933348 E-174
+x^218*  3.2586120424601915408821719193404432 E-176
+x^220*  2.9680473428374280896879078486321870 E-178
+x^222*  2.6789814810639341414957466989183588 E-180
+x^224*  2.3964300066679809069756609257598127 E-182
+x^226*  2.1246663171265192034320134663780201 E-184
+x^228*  1.8671613336865434920788141519694894 E-186
+x^230*  1.6265659181520794885171318855735103 E-188
+x^232*  1.4047311124567213777863504355051806 E-190
+x^234*  1.2027597222569246629766702724383645 E-192
+x^236*  1.0210818811257405412205760707438274 E-194
+x^238*  8.5954698330499079592176872001434089 E-197
+x^240*  7.1752467396949083627525669755611112 E-199
+x^242*  5.9400831951367278977066109854343018 E-201
+x^244*  4.8771541485230735320907995716303265 E-203
+x^246*  3.9718058841925562252766251048314798 E-205
+x^248*  3.2083811838309789218291913075664471 E-207
+x^250*  2.5709207542651528481611339463608225 E-209
+x^252*  2.0437328270221585277476758986377295 E-211
+x^254*  1.6118318268018194488610632610936588 E-213
+x^256*  1.2612539851622949493154153063528759 E-215
+x^258*  9.7926270498507619328506410212243514 E-218
+x^260*  7.5445948936227155202620182841491190 E-220
+x^262*  5.7681758076228812615679592722215705 E-222
+x^264*  4.3765542601429201889262169463408459 E-224
+x^266*  3.2956602838905411738805382291241816 E-226
+x^268*  2.4631649944977078860504830835597113 E-228
+x^270*  1.8272997644208747788789532851884910 E-230
+x^272*  1.3455977312437524650106651013022332 E-232
+x^274*  9.8363374946541029880070340468144678 E-235
+x^276*  7.1381809615231028869961088518181511 E-237
+x^278*  5.1428106729307704472070264913462643 E-239
+x^280*  3.6787047629802064484011221342722680 E-241
+x^282*  2.6127207267821731473382386378136625 E-243
+x^284*  1.8425387570385804039507331118314404 E-245
+x^286*  1.2902904340009841445214910884150782 E-247
+x^288*  8.9727763846206630162980372068308648 E-250
+x^290*  6.1966326997131589119294793979444780 E-252
+x^292*  4.2500588021933414188786367446496555 E-254
+x^294*  2.8951085088163651685307592608650410 E-256
+x^296*  1.9587799296431179662082428708715490 E-258
+x^298*  1.3163677559370068573836516830365380 E-260
+x^300*  8.7873767778245600528981456643467563 E-263
+x^302*  5.8270809589890229808612834844831363 E-265
+x^304*  3.8385902960181587756238432788550292 E-267
+x^306*  2.5121191692848013794533790420901500 E-269
+x^308*  1.6333343564851485478337357735030594 E-271
+x^310*  1.0551026532424685686756552712554848 E-273
+x^312*  6.7720043527468723160507297423038065 E-276
+x^314*  4.3187746458416666065524473697031941 E-278
+x^316*  2.7367954321478776213957675264172161 E-280
+x^318*  1.7233760862141576032030570595919383 E-282
+x^320*  1.0784275050796011077982183929187538 E-284
+x^322*  6.7064398959285544549913919978797548 E-287
+x^324*  4.1447669204107379871157734111016636 E-289
+x^326*  2.5458439895971144620924646826388123 E-291
+x^328*  1.5541876515984266195742476223477542 E-293
+x^330*  9.4304260565984170069476612148378349 E-296
+x^332*  5.6876304211685272334771942063474693 E-298
+x^334*  3.4097255681881316920313209049478022 E-300
+x^336*  2.0319410656792319220244794928747544 E-302
+x^338*  1.2037099398982330855716119487558144 E-304
+x^340*  7.0887048796743225068464281718891817 E-307
+x^342*  4.1501264333709663750357784755238772 E-309
+x^344*  2.4155722872164402402366408421263890 E-311
+x^346*  1.3978409273259589099340096795576327 E-313
+x^348*  8.0424620773383190885938430521414217 E-316
+x^350*  4.6007457555223156980844993425732100 E-318
+x^352*  2.6169149686347582147940021505008903 E-320
+x^354*  1.4800868978782196236759264628558889 E-322
+x^356*  8.3240550670500646398165455766546783 E-325
+x^358*  4.6552879444777486212573378176444272 E-327
+x^360*  2.5890212733452180092493549562050644 E-329
+x^362*  1.4319095881582849186553262993046333 E-331
+x^364*  7.8758924446023022551600836551281423 E-334
+x^366*  4.3082540491380464458304335127991022 E-336
+x^368*  2.3438681934420224431366294599560156 E-338
+x^370*  1.2682599601734130141997140486447331 E-340
+x^372*  6.8255761585313529030023214624502090 E-343
+x^374*  3.6537504179850186529808586971817157 E-345
+x^376*  1.9454470640383043345151522063744741 E-347
+x^378*  1.0303701228040713220056802251251407 E-349
+x^380*  5.4284092933935871432575826027630447 E-352
+x^382*  2.8449161093678520514495836019835479 E-354
+x^384*  1.4831867268736039429035720562759859 E-356
+x^386*  7.6924289606812687736387887303841743 E-359
+x^388*  3.9690278432912187579900658177233798 E-361
+x^390*  2.0373674015999579024215378976211578 E-363
+x^392*  1.0404724062589859131137824355131333 E-365
+x^394*  5.2866324949365548094078654832688881 E-368
+x^396*  2.6725524292132869175146706128752359 E-370
+x^398*  1.3442591789564729820372553297261899 E-372
+x^400*  6.7276043932161689622055463797647739 E-375
+x^402*  3.3501898805653338209475704099873953 E-377
+x^404*  1.6600482741408619782632712400525639 E-379
+x^406*  8.1851190380409222601743302375742820 E-382
+x^408*  4.0159919520697047756962795777863189 E-384
+x^410*  1.9608062398059577710626370130100980 E-386
+x^412*  9.5271038245275776436955979522559012 E-389
+x^414*  4.6066131783811398794367271936266436 E-391
+x^416*  2.2167023535320304489442025975165265 E-393
+x^418*  1.0615681256049270688335788443041634 E-395
+x^420*  5.0595649367762156018326402207801767 E-398
+x^422*  2.4000106325239728954321198912425041 E-400
+x^424*  1.1330722982063571446658985926135453 E-402
+x^426*  5.3242228851643212061132930170268939 E-405
+x^428*  2.4901102609759374945796280980499327 E-407
+x^430*  1.1591888011476562915623901579261002 E-409
+x^432*  5.3712138150299921355619109397593612 E-412
+x^434*  2.4773233699268207877862835462388887 E-414
+x^436*  1.1373501682539365954984102549761850 E-416
+x^438*  5.1977584052351058915934485956926015 E-419
+x^440*  2.3645987423128675227958881955755498 E-421
+x^442*  1.0708466527742741848235811063637934 E-423
+x^444*  4.8276356812403859567097281296510472 E-426
+x^446*  2.1666457058982492763117755485891586 E-428
+x^448*  9.6804661647641518073467320429143415 E-431
+x^450*  4.3059429864857946197237910230468715 E-433
+x^452*  1.9068323395955325335591466711305134 E-435
+x^454*  8.4069312021948624849915116107499447 E-438
+x^456*  3.6902150407655958254380893563093443 E-440
+x^458*  1.6127365347504055955069556099905408 E-442
+x^460*  7.0174777108830590245620859325682571 E-445
+x^462*  3.0402741961576603620194757027175907 E-447
+x^464*  1.3114952288328110327887496287280118 E-449
+x^466*  5.6331460547306824417499990924856986 E-452
+x^468*  2.4092044658830599569385868148282707 E-454
+x^470*  1.0259887676107284313384139546012225 E-456
+x^472*  4.3507660565262587322577700129128264 E-459
+x^474*  1.8371763626014710809425480702452461 E-461
+x^476*  7.7251283884274221825716125421795202 E-464
+x^478*  3.2347296144298196095574184027015663 E-466
+x^480*  1.3488240565723773570312053678447237 E-468
+x^482*  5.6009954020913014783199103742920378 E-471
+x^484*  2.3161955123320472025791616310545782 E-473
+x^486*  9.5387778001852536493119022676929455 E-476
+x^488*  3.9122368065366132896010583186772364 E-478
+x^490*  1.5980108789305212015372959236632205 E-480
+x^492*  6.5007546043738560791390486017894067 E-483
+x^494*  2.6338100282375868123612084089514439 E-485
+x^496*  1.0627934950764545348587435908647804 E-487
+x^498*  4.2713402573825864197269787671449632 E-490
+x^500*  1.7097684889342256728475583262169178 E-492
+x^502*  6.8167170690893784857460923010713081 E-495
+x^504*  2.7069794280609758907801550453849263 E-497
+x^506*  1.0707133556414323052704577339192125 E-499
+x^508*  4.2183912744938588321093927922482137 E-502
+x^510*  1.6554368545070885644250347049640618 E-504
+x^512*  6.4710862686884100831842289143927412 E-507
+x^514*  2.5196905612142926143864290147382080 E-509
+x^516*  9.7730307507476447488074147080763105 E-512
+x^518*  3.7759818172532326284300013425827017 E-514
+x^520*  1.4533010325546245227702971437250612 E-516
+x^522*  5.5720212566576728141525208361883109 E-519
+x^524*  2.1281774067436817198358112855254375 E-521
+x^526*  8.0974283036088274826054118150946671 E-524
+x^528*  3.0692827850287978024970428231289811 E-526
+x^530*  1.1590000294229090885478486724562258 E-528
+x^532*  4.3600647297857883309324682001259171 E-531
+x^534*  1.6340732544631760993127019842572065 E-533
+x^536*  6.1013410862961239302394223399514090 E-536
+x^538*  2.2696575831980264118461130365373784 E-538
+x^540*  8.4116781099102540484574194196715117 E-541
+x^542*  3.1059766267867071243511144656800518 E-543
+x^544*  1.1426490927215579200366990108987664 E-545
+x^546*  4.1882501517153093051205893495243755 E-548
+x^548*  1.5295483910899009974549524278253060 E-550
+x^550*  5.5655808938547381233123139109469887 E-553
+x^552*  2.0178096700795144737589310032739830 E-555
+x^554*  7.2891691901754626842544881489312288 E-558
+x^556*  2.6236727486199870277808066484473897 E-560
+x^558*  9.4098072723088711513345530197835567 E-563
+x^560*  3.3627672674347540093336157394946285 E-565
+x^562*  1.1974667459520276815693244749893372 E-567
+x^564*  4.2489946917606168696332967261626913 E-570
+x^566*  1.5023477183994287175822262696270016 E-572
+x^568*  5.2932415495228858569415743927490860 E-575
+x^570*  1.8584262661552812916649197258771482 E-577
+x^572*  6.5019957821473725533639805524051144 E-580
+x^574*  2.2668932860841137891509077343544729 E-582
+x^576*  7.8759653025974339712690776027586565 E-585
+x^578*  2.7269054608787705338470784012530895 E-587
+x^580*  9.4088197056789576557674817839914784 E-590
+x^582*  3.2352225499222988825731449809832761 E-592
+x^584*  1.1086188574907933487350934313471123 E-594
+x^586*  3.7859471734049936555531848945534595 E-597
+x^588*  1.2885049527100059791377750041493628 E-599
+x^590*  4.3704073815830744250845512499365575 E-602
+x^592*  1.4773623750012737580279726250722068 E-604
+x^594*  4.9772149455130772543535991336437389 E-607
+x^596*  1.6711865764110440014395419009948120 E-609
+x^598*  5.5925201598622233712757741494808079 E-612
+x^600*  1.8652589001443449088121380514937168 E-614; }

It turns out that you lose a lot of precision in that multiplication by exp(-x^2), and you need an extra large amount of precision in the entire function to generate accurate results at the real axis, where the derivatives are cancelling. I generated a 300 term (x^600) asymptotic for the entire function, in pari-gp using 134 decimal digits precision and a lot of tricks, which gives very good results if $|x|<=9$, and has its best accuracy of about 35 decimal digits for values near x=+9. Updated to include the first 125 terms of entiref (x^250), for Antonio Vargas.

{entiref=
+x^ 2*  1.000000000000000000000000000000000000000000000000000000000000000000
+x^ 4* -0.3903080328022398631614181122699925379762414278275504252183985648850
+x^ 6*  0.1375788748110124682424374847928600715147226065003467931915901836882
+x^ 8* -0.03863728761483880499259264298678193713104655257223268868775160299498
+x^10*  0.008747481601094830079665989070608193777827806357836361521383464628533
+x^12* -0.001641193268278966279231353709768925284150022492905004523264592289770
+x^14*  0.0002616639105782442949733463357922654190354746495134737600001108244232
+x^16* -0.00003618561358737824400665656054216919436248417993097722079578770229568
+x^18*  0.000004412510097357058845738729777072243729802120876650229561000776190015
+x^20* -0.0000004808126873165497443393515090015363089752813947730516960136053584884
+x^22*  0.00000004733084424489864100234363177603646208282049355730062960571981692851
+x^24* -0.000000004247428934267281201811125771560623360590507330764270562145091149168
+x^26*  0.0000000003501335594761328997744701771923262297783910556272608750113648130919
+x^28* -2.668630243903328175021565547870923863178634487323350804813312708238 E-11
+x^30*  1.891120738388056412638527127164007962138666819660099591006583816498 E-12
+x^32* -1.252097029553170339693678466000739519438205823046719982007820130696 E-13
+x^34*  7.778504706304949680843875697095710856189980020607659383183493741077 E-15
+x^36* -4.551238453526195314455658434004708125426827807197877960382484343355 E-16
+x^38*  2.516481981763348464137143471704196452796661296427446064374946665221 E-17
+x^40* -1.318838800154599762565175454315150405570382986212012474812519462672 E-18
+x^42*  6.568953128927681260495586323443395623814643892064506599693955639215 E-20
+x^44* -3.117227042071800769320929752367637475934420891851486694342979093797 E-21
+x^46*  1.412450547402984869528281506638892938809787199205571323311490434722 E-22
+x^48* -6.123381116921861262956124159360437736265928988389993129732737773670 E-24
+x^50*  2.544659298608104031544788552184204651343521455381910179877458046177 E-25
+x^52* -1.015384688950120000418048348842714390022585963928704523014715143804 E-26
+x^54*  3.896540198116138343915535799437739159761006073575164646517770124122 E-28
+x^56* -1.440155194951485402851719427157641346171222033695983959825656994082 E-29
+x^58*  5.133451976996463967204245633684011099607434720643460675804128943736 E-31
+x^60* -1.766963080358872061808820431113781079212151015127724863334759364894 E-32
+x^62*  5.879957458426308562941552764240728133869531432107574746089012733808 E-34
+x^64* -1.893770931532020419347879666434363020192223533099907670187271416789 E-35
+x^66*  5.909286356660390400719055048074241546421263606696828768530587540150 E-37
+x^68* -1.788205488532361192026140779288595376895462864177359532326866558905 E-38
+x^70*  5.252549951221843928744208387464144058077763922035517888655816841696 E-40
+x^72* -1.498877612875142166111638892660063056650438790686374124478266494435 E-41
+x^74*  4.158697751492591570701921665524932295810396287244992772370050923038 E-43
+x^76* -1.122733248293364879313692096444812328194682894662060194420057474181 E-44
+x^78*  2.951474664352473043243621851555023598586562525517034340782506655576 E-46
+x^80* -7.560381699931795105957872349521925165356917620746234113693051018497 E-48
+x^82*  1.888315095977826644356334460543477687577192792001237225453836530092 E-49
+x^84* -4.601519242339140903411470798077886447301646630443349834596685081384 E-51
+x^86*  1.094664590132602738233248736166351745407731167142735995520989816639 E-52
+x^88* -2.543659239935988987323687867118984800714268535442454864259414800107 E-54
+x^90*  5.776550852534097828063545564188677119116262766430314568849842566158 E-56
+x^92* -1.282724525873545450951998309430456675494571718070986174948894664498 E-57
+x^94*  2.786550193951782813731959474458748346686496730200934398002234067872 E-59
+x^96* -5.924796219155865282227842097252363371054756731897868316742816486519 E-61
+x^98*  1.233527598068494418478206898158068983739087998551787680776846585950 E-62
+x^100* -2.515828404089953602462783677019212534380186712868882793528751556938 E-64
+x^102*  5.028634635634635008744667900948918019719316369147515442777126677752 E-66
+x^104* -9.854377284524946586809705153062349158016923826451027438638680704156 E-68
+x^106*  1.894020794459041649233501503643638549654984182060763083403674644191 E-69
+x^108* -3.571715448187869017003660008491485852335273139385049810222209685905 E-71
+x^110*  6.610885668937218433636583251362803845963683712597608164045899701800 E-73
+x^112* -1.201383379358396549358396599277114972543590386777379893376204543694 E-74
+x^114*  2.144301897694357753849949532313598104311515452213075452235661194144 E-76
+x^116* -3.760197738542761504349012636731574507882432263835309447110166260639 E-78
+x^118*  6.480211741319777721506940780334859206917269907132702701374768595357 E-80
+x^120* -1.097868281975536647476825739658407890750814197669154769775078777018 E-81
+x^122*  1.829019186336640763717497618107649513284335060532279553384072569044 E-83
+x^124* -2.997185442007957902702255983085528252764017336031211586721978637903 E-85
+x^126*  4.832287249211750743130871742153693008721659847726999190499792583396 E-87
+x^128* -7.667404443833738779687211241317443776638179802563035782248565648430 E-89
+x^130*  1.197594211834971661615768085254854409781140178224319935510810381346 E-90
+x^132* -1.841800121066242705763750022924912477866020151481708619298940324307 E-92
+x^134*  2.789647834547488270647010060855645486069515549386949780678721377725 E-94
+x^136* -4.162266125651759603838113604752964602812785195883350482682973634581 E-96
+x^138*  6.118999928317925481381192225890675148751814682862176032132511685791 E-98
+x^140* -8.865330678999651962835842153809433512571656071188619587628444030945 E-100
+x^142*  1.266089402368549116274553460307709410669002303665106208587842573965 E-101
+x^144* -1.782695740143867873437288634604271606513376821527396654187775895607 E-103
+x^146*  2.475252482719049941043718770785739933970570266173685592446539353333 E-105
+x^148* -3.389806009329760011338071355458736319624926191377945998968469025360 E-107
+x^150*  4.579569069924557790655710703626545669844045386426415160598538821559 E-109
+x^152* -6.104470428009609822921799387860516632083401016101646593127326435358 E-111
+x^154*  8.030120784376878888277868900960847644398410169255398495764193696587 E-113
+x^156* -1.042610134551399132512192894393815327046391746322889796866702135867 E-114
+x^158*  1.336351578022676496857310907413228385903927174260656211952637088693 E-116
+x^160* -1.691179581529598239426505961094848974095760488267751890934704843814 E-118
+x^162*  2.113481356639418507523393435239304265561613287102718187711205539631 E-120
+x^164* -2.608642602922945430190643377859337450604490222044918791917429876137 E-122
+x^166*  3.180565311188874704400731660018734486677248611303935372478838184371 E-124
+x^168* -3.831176268162866654258028516201433015330532910593557502244918037286 E-126
+x^170*  4.559959587154329884447303096751726156363626505427486746883897121085 E-128
+x^172* -5.363550484670114486210135555689694028460834474800577789962082243971 E-130
+x^174*  6.235428590579867957343880554136586388661181914747361574379258452075 E-132
+x^176* -7.165746786945795031819109866411455308402351144826436116534993531665 E-134
+x^178*  8.141325694844154932777542849523038027962202313355128923098361810177 E-136
+x^180* -9.145834621213140295546426526058350985317274495867861820244190433437 E-138
+x^182*  1.016016758873666168137674419744367184273824198099688156033734387316 E-139
+x^184* -1.116300892163672937822054959110786794772247338530147368851902637815 E-141
+x^186*  1.213156796261082896807017173463902988150142443594812268429758514907 E-143
+x^188* -1.304244823612350581190573305983323593444872082771407080444115051944 E-145
+x^190*  1.387260415633070338173861653090224062526397102218872646793991014657 E-147
+x^192* -1.460032946832601764384397606070254320663937057536628357514957888424 E-149
+x^194*  1.520621698147030320942480529357215787848927247364798485552564259890 E-151
+x^196* -1.567402936447916445518062782482952797659104724392302868396510919510 E-153
+x^198*  1.599142590243069005840412857858230023443640259186072246283081880194 E-155
+x^200* -1.615049974865374525324446298458064185449641967351391907754252767769 E-157
+x^202*  1.614809347225369443401178440491063229228506783818353884475292152412 E-159
+x^204* -1.598587638641597838416338717718013191319752618551290751592895189296 E-161
+x^206*  1.567018381237595627790660836427084572271700183681831983415112355905 E-163
+x^208* -1.521163460405393133572428306362547480589249227627058448510870380932 E-165
+x^210*  1.462455754914626109611601044814563792166256742195578989220159675485 E-167
+x^212* -1.392626853904929797232369137876299060036183274965532509436234107550 E-169
+x^214*  1.313624787613578141311218281013625922487550082252272718032620949850 E-171
+x^216* -1.227527038297800048772159791915756614723370420532870183476386455889 E-173
+x^218*  1.136454012959874208148623110555250243882468587068188799047846697127 E-175
+x^220* -1.042487701441857501301724217917044246507724683790098449808805153588 E-177
+x^222*  9.475994836566105827163963474618701613363502964054983154895033021797 E-180
+x^224* -8.535900796867395118239675564567240113013866634629565297247051801656 E-182
+x^226*  7.620435566888528732587262945881461355615364188168018494681378205898 E-184
+x^228* -6.742962156052605653465247242782900478399607118486323600778115878165 E-186
+x^230*  5.914201660842093307918061796648192870207101608569674977191463823864 E-188
+x^232* -5.142205291645442978199722486626833630695632032714937000525401568451 E-190
+x^234*  4.432445314692917766797298545381492675209416947071904463598868148333 E-192
+x^236* -3.788002957847998154329913053953521334353247152046908752442516893891 E-194
+x^238*  3.209828928573606792040557996069003013738657251455883902848691693262 E-196
+x^240* -2.697051812126019376626441440153706864478218697120028713023328245966 E-198
+x^242*  2.247310949602852854399680135921575340385709404131169396926516164309 E-200
+x^244* -1.857093042637235093239464604077596885256395737929052158111973063832 E-202
+x^246*  1.522055261711685438716128750383578864561437143026729427808420822703 E-204
+x^248* -1.237321624830459170392415702769679123777155675244895323971131421559 E-206
+x^250*  9.977434841349342154444290231168740753643009928091675955912339581227 E-209}
• This is really interesting. Would you be able to include the first ~150 coefficients of entiref as well? Sep 5 '14 at 21:23
• @AntonioVargas I updated to include terms to x^200 for the entiref function. Originally I tried posting x^600, but was over the character limit for mathstack... Sep 5 '14 at 22:48
• Thanks, I appreciate it. I was interested in them for numerically plotting the zeros of the partial sums of the power series. Sep 5 '14 at 23:00
• btw; this particular solution is exactly equivalent to interpolating $f(x)\approx e^x\ln(x+1)$. And then the asymptotic solution is $e^{-x}f(x)$. But I wonder if some smaller base would work equally well or better, instead of $e^x$, could we use $1.1^x$ or $1.01^x$, and get reasonably good results with fewer Taylor series coefficients and better convergence for entiref. Sep 6 '14 at 12:16
• @AntonioVargas I think maybe I can find a closed form solution for the Taylor series coefficients of $f(x)\approx e^x\ln(x+1)$, or a limit equation, for the larger coefficients. I could also post some pretty pictures showing where the zeros are, near the imaginary axis, spread apart by approximately $2\pi i$ Sep 7 '14 at 17:57

As said in the comments I will write a partial "answer". I do this because its to much for a comment and I feel its not an edit to the question. I might edit the "answer" a few times.

I believe there is a real entire $f(x) = O(ln(x^2+1))$ where we use big-O notation.

Here is the assumed "proof sketch" :

$f(x) = \sum_{n=1}^\infty a_n x^{2n} = O(ln(x^2+1))$

Where $a_n$ alternates sign starting with $a_1 < 0$.

Also $|a_{2n}| > |a_{2n-1}|$.

$x > 0 \implies a_{2n-1}x^{4n-2} + a_{2n} x^{4n} < O(ln(x^2+1))$

$x > 1 \implies ln(a_{2n-1}x^{4n-2} + a_{2n} x^{4n}) < O(ln(ln(x^2+1)))$

$x > 1 \implies (4n-2) ln(x) + ln(a_{2n-1} + a_{2n} x^2) < O(ln(ln(x^2+1)))$

Now relaxing the equation :

$x > 1 \implies (4n-2) ln(x) + ln(a_{2n-1}) + ln(1 + A_n x^2) = C(ln(ln(x^2+1)))$

$x > 1 \implies ln(a_{2n-1}) = C(ln(ln(x^2+1))) - (4n-2) ln(x) - ln(1 + A_n x^2)$

Now $a_{2n-1} = exp ( min ( C(ln(ln(x^2+1))) - (4n-2) ln(x) - ln(1 + A_n x^2) )$

The minimum must occur where the derivative is $0$.

Let the minimum be attained at $x= y$ then

$a_{2n-1} = exp( C(ln(ln(y^2+1))) - (4n-2) ln(y) - ln(1 + A_n y^2)$

SO we must compute $y$ by setting the derivative to 0 and solving for x.

I use an approximation of the derivative ;

$\frac{d}{dx} C(ln(ln(x^2+1))) - (4n-2) ln(x) - ln(1 + A_n x^2) =$ ( approx )

$-(4n-2)/x + C/(x ln(x))$

So we need to solve : $(-(4n-2) + C/ln(x))/x = 0$.

Or equivalently : $C = (4n-2) ln(x)$

$y = x = exp(C/(4n-2))$

(end proof sketch)

Numerically it seems pretty good.

I must thank tommy1729 and Sheldon (https://math.stackexchange.com/users/43626/sheldon-l)

I got the inspiration from them at the tetration forum : http://math.eretrandre.org/tetrationforum/showthread.php?tid=863

(they call this " fake function theory " )

I also wonder about least squares methods. And integral representations.

This does not constitute a proof , I know. But its too big for a comment.

EDIT :

Actually this might be a disproof ?? It seems the sequence $a_n$ might not be shrinking fast enough to give an entire function ? Or did I make a big mistake ?

But the sine function seems to suggest alternating sequences are very powerfull in approximating.

Is this the wrong way to compute it ? Did I make wrong assumptions ? A bit confused.

EDIT 2 :

Tommy suggested finding an asymptotic to $f(x) = ln(x^2+1) exp(x^2)$. The solution would then be $f(x) exp(-x^2)$. That might work.

• Tommy's suggestion for interpolating the even function: $g(x)\approx (\ln(x^2+1)\exp(x^2))$ works amazingly well! Using this approximation, then $\ln(x^2+1)\approx f(x)\;\;\; f(x) = g(x)\exp(-x^2)$. I will post equations and the Taylor series I generated later. Sep 3 '14 at 11:44