Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

What are some surprising equations / identities that you have seen, which you would not have expected?

This could be complex numbers, trigonometric identities, combinatorial results, algebraic results, etc.

I'd request to avoid 'standard' / well-known results like $ e^{i \pi} + 1 = 0$.

Please write a single identity (or group of identities) in each answer.

I found this list of Funny identities, in which there is some overlap.

share|cite|improve this question

closed as too broad by quid, Eric Wofsey, S.Panja-1729, Zachary Selk, ᴡᴏʀᴅs Oct 28 '15 at 1:13

There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs.If this question can be reworded to fit the rules in the help center, please edit the question.

56  
I really can't believe no one has posted this yet: xkcd.com/687 – mikeTheLiar Sep 26 '13 at 14:48
20  
This is not in line with what you are looking for, but as a child I discovered that 10million pi is the number of seconds in a year to 1/2% accuracy. This is useful for quick back of envelope calculations, where seconds are involved. – JoeTaxpayer Sep 26 '13 at 19:48
11  
Pi seconds is a nanocentury! – Oscar Cunningham Oct 1 '13 at 21:59
    
@CalvinLin Is that until you get the most rare badge? I got the 81st favorite too! – zerosofthezeta Oct 2 '13 at 4:32
1  
The three trigonometric identities in the following exercises of my Wikibook: en.wikibooks.org/wiki/On_2D_Inverse_Problems/… – DVD Oct 12 '15 at 2:51

92 Answers 92

If $a,b,c,d$ are in arithmetical progression, then $$\frac{d^2-a^2}{c^2-b^2}=3.$$

share|cite|improve this answer
    
Do you mean 9 instead? – Calvin Lin Sep 17 '14 at 12:29
    
No… e.g. $1,2,3,4$ are in arithmetical progression, and $(4^2-1^2)/(3^2-2^2)= 15/5 = 3$. – Kieren MacMillan Sep 17 '14 at 13:32
    
Write $(a,b,c,d) = (a,a+\delta,a+2\delta,a+3\delta)$, and by substitution \begin{align} \frac{(d^2 - a^2)}{(c^2 - b^2)} = \frac{(d-a)(d+a)}{(c-b)(c+b)} &= \frac{(3\delta)(2a+3\delta)}{(\delta)(2a+3\delta)} = 3. \end{align} – Kieren MacMillan Sep 17 '14 at 13:37
    
@CalvinLin: Surprising, yes? =) – Kieren MacMillan Sep 17 '14 at 20:56
    
Oh sorry, I completely misread it as (d-a)^2 / (c-b)^2. Not sure why. Yes, this is initially surprising :) – Calvin Lin Sep 17 '14 at 21:35

Pfister's 16-Square Identity:

$$(x_1^2+x_2^2+x_3^2+\dots+x_{16}^2)(y_1^2+y_2^2+y_3^2+\dots+y_{16}^2) = z_1^2+z_2^2+z_3^2+\dots+z_{16}^2$$

where the $z_i$ are rational functions of the $x_i, y_i$. One would have thought that $n$ square identities are only for $n = 1,2,4,8$, but non-bilinear ones in fact are for all $n = 2^m$.

share|cite|improve this answer

Another one, which occured to me when I began to learn about double-sums in the context of divergent summation. I really had to chew on this, that the sum of the vertical sums can be different from the sum of the horizontal sums... And just different by the exact value of 1. So this had some appeal as another example of Where is the missing 1 in the equation? (From an older essay of mine):

enter image description here

share|cite|improve this answer
3  
It's nice to see a pair of natural computations with divergent sums that do not miraculously coincide. Reading Euler's work, as one can in Lagarias's article in the Bulletin right now, you get the impression that there is a kind of mystic unity to the spectrum of cleverly done sums so that when done "right" they reveal some consistency in our aesthetic choices in extending math. Fortunately not. – Ryan Reich Sep 26 '13 at 21:55
    
Is there a simple proof explaining why the difference is always one? Or a name of the result that one can google? – Aaron Sep 27 '13 at 16:50
    
I don't have it at hand, but it can be reconstructed when one does Ramanujan-summation or uses the Euler/MacLaurin-formula and replaces the occuring Bernoulli-numbers by zetas. In the Ram.-summation we have one additional integral for such sums, and in the Euler/MacLaurin occurs the Bernoulli-number $B_0$ which can be understood as renormalized ratio of $\zeta(1)/\Gamma(0)$ equalling 1 (or -1). In the above formula would the latter idea occur with an additional row above the main-matrix with the series of $1+1/2+1/3+...=\zeta(1)$ and denominator of $(-1)!$ , whose ratio is normalized $-1$ – Gottfried Helms Sep 27 '13 at 17:29

This bit of notational juggling may cause one do double take...

$$\huge \sqrt[\sqrt{2}]{2} = \sqrt{2}^\sqrt{2}$$

By definition, the LHS is the number $x$ such that $x^\sqrt{2} = 2$. It is simple to check that the RHS also has this property.

share|cite|improve this answer

$$\sum_{n=1}^\infty(n\,\operatorname{arccot}n-1)=\frac12+\frac{17\,\pi}{24}-\ln\sqrt{e^{2\pi}-1}+\frac1{4\pi}\operatorname{Li}_2\frac1{e^{2\pi}},$$ where $\operatorname{Li}_2$ is the dilogarithm.

share|cite|improve this answer

Ramanujan stated this radical in his lost notebook:

$$\sqrt{5+\sqrt{5+\sqrt{5-\sqrt{5+\sqrt{5+\sqrt{5+\sqrt{5-\dots}}}}}}} = \frac{2+\sqrt 5 +\sqrt{15-6\sqrt 5}}{2}$$

I still don't have any idea on this one.

share|cite|improve this answer
    
Seen this? mathworld.wolfram.com/NestedRadical.html – Bennett Gardiner Oct 2 '13 at 23:24
1  
@BennettGardiner there is nothing given on the above radical?? – SHOBHIT GAUTAM Oct 4 '13 at 10:21
1  
True, I missed the negatives, what is the pattern for the minus signs? – Bennett Gardiner Oct 4 '13 at 12:07
3  
@BennettGardiner ++-+++-++++-+++++-.... – SHOBHIT GAUTAM Oct 4 '13 at 12:12
    
Wow. Has anyone proven this? Or is this a Ramanujan special? – Bennett Gardiner Oct 5 '13 at 3:10

This one is one of my favorite:

$$ \log(1+2+3) = \log(1)+\log(2)+\log(3) $$

share|cite|improve this answer
    
Can you generalize it? – Abramo Dec 12 '13 at 19:58
3  
$\log(1+1+2+4)=\log(1)+\log(1)+\log(2)+\log(4)$ The general case is analyzed in www-users.mat.umk.pl/~anow/ps-dvi/si-krl-a.pdf – Mark S. Dec 13 '13 at 3:23
1  
Your equation is true because $1+2+3=6$, and $1\times 2\times 3=6$. So, $\log(1+2+3)=\log(1\times 2\times 3)=\log(1)+\log(2)+\log(3)$ (because $\log(x+y+z)=\log(x)+\log(y)+\log(z)$) – JChau Feb 28 '14 at 2:29
    
Ok JChau, I knew about this! ;-) – Abramo Mar 1 '14 at 14:55

A rather simple one $$2^4 = 4^2$$

You can use this one to "proof" (as a prank) that $x^y = y^x$ ;-)

share|cite|improve this answer
5  
There are not other integer solutions (except for $x=y$). – pts Sep 26 '13 at 11:02
3  
@pts There are infinitely many rational solutions, though, with $(e,e)$ as a limit point. – A Walker Sep 27 '13 at 5:25

There are many fantastic equations that I've seen, but the one that definitely sticks out as top in my mind is the Atiyah-Singer Index theorem (it can be written in many ways; this is the way that I first learned it).

$$\operatorname{Ind}(D) = (-1)^n \int_M \frac{\operatorname{ch}(E)-\operatorname{ch}(F)}{\operatorname{e}(TM)} \operatorname{Td}^{hol}(TM \otimes \mathbb C)$$

Here $M$ is a $2n$-dimensional smooth compact manifold, with $E$, $F$ vector bundles over $M$, $D:\Omega^0(E) \rightarrow \Omega^0(F)$ is an elliptic differential operator, $\operatorname{Ind}(D) = \dim \ker D - \dim \operatorname{coker} D$, $\operatorname{ch}$ denotes the Chern class, $\operatorname{e}$ is the Euler class, and $\operatorname{Td}^{hol}$ is the holomorphic Todd class. Note that in this formulation the choice of the divisor depends naturally on the choice of $D$, a fact which is obscured in the notation.

It's probably the only equation I've ever seen in math which forced me to think about two entire fields in a different way. The mere fact that something like this connecting analysis and topology at such a deep level could be true is really incredible.

share|cite|improve this answer

1. $$ e^{\pi i} + 1 = 0$$

This simple equation links five fundamental mathematical constants:

  • The number 0, the additive identity.
  • The number 1, the multiplicative identity.
  • The irrational number π (pi), pivotal in trigonometry and geometry.
  • The transcendental constant e, the base of the natural logarithm, widely used in scientific analysis.
  • The number i (iota), the imaginary unit of complex numbers, and the square root of -1.

Moreover, the three basic arithmetic operations occur exactly once each: addition, multiplication and exponentiation; and these are magically wound into one single relation(=).

The beauty lies in the fact that an irrational number, raised to the power of an imaginary number multiplied with another irrational number, exactly becomes zero when added to 1.

As quoted by Benjamin Peirce, a noted American 19th-century philosopher,mathematician, and professor at Harvard University, "it is absolutely paradoxical; we cannot understand it, and we don't know what it means, but we have proved it, and therefore we know it must be the truth."

This identity is a special case of Euler's Formula: $$e^{ix}=cosx+ i sinx$$ It's almost mystical that these values are even related to one another.


2. The solution to this equation: $$1+\frac{1}{\phi}=\phi$$ Which is The golden ratio:$$\phi=\frac{1+\sqrt5}{2}=1.6180339887 . . .$$Which can turn into recurrence equation: $$\phi^{n+1}=\phi^n+\phi^{n-1}$$ Beautiful how it is also related to Fibonacci numbers: $$1 , 1 , 2 , 3 , 5 , 8 , 13 , 21 , ...$$ Where if you divide any consecutive Fibonacci numbers, in the infinite horizon will converge to, again, the golden ratio: $$\lim_{n\to\infty}\frac{F(n+1)}{F(n)}=\phi$$


3. Tupper's Self Referential Formula

enter image description here

When plotted with k=960939379918958884971672962127852754715004339660129306651505519271702802395266424689642842174350718121267153782770623355993237280874144307891325963941337723487857735749823926629715517173716995165232890538221612403238855866184013235585136048828693337902491454229288667081096184496091705183454067827731551705405381627380967602565625016981482083418783163849115590225610003652351370343874461848378737238198224849863465033159410054974700593138339226497249461751545728366702369745461014655997933798537483143786841806593422227898388722980000748404719

$0 \le x \le 106$ and $k \le y \le k + 17$, the resulting graph looks like this:

enter image description here


4. Ramanujan's golden ratio equation:enter image description here


5. Gaussian integral:

$$\int_{-\infty}^\infty \! e^{-x^2}dx = \sqrt{\pi}$$


6. Cauchy's Integral Formula: $${f^{\left( n \right)}}\left( a \right) = \frac{{n!}}{{2\pi i}}\oint_\gamma {\frac{{f\left( z \right)}}{{{{\left( {z - a} \right)}^{n + 1}}}}dz}$$ The derivative of a analytic function given as a closed path integral in the complex plane.


7. Ramanujan's Infinite series for calculation of $\pi$. It converges faster

$$ \frac{1}{\pi} = \frac{2\sqrt{2}}{9801} \sum^\infty_{k=0} \frac{(4k)!(1103+26390k)}{(k!)^4 396^{4k}}$$


8. Batman Curve

The batman curve is a piecewise curve in the shape of the logo of the Batman superhero originally posted on reddit.com on Jul. 28, 2011. It can written as two functions, one for the upper part and the other for the lower part, as: enter image description here


9. The Schrodinger Equation:

$$H\Psi(x,t) = i\hbar\frac{\partial}{\partial t}\Psi(x,t)$$

share|cite|improve this answer
2  
Thanks! This is a good list :) – Calvin Lin Oct 2 '13 at 4:00
    
I would rewrite the 2. formula to make it even more symmetric/astonishing/mystical as $$ \phi^{-1}=\phi-1$$ – Gottfried Helms Nov 19 '13 at 10:14
1  
@GottfriedHelms Even better as $$\phi^1-\phi^0=\phi^{-1}$$ – Kristoffer Ryhl Sep 20 '14 at 9:58

If $x^n + y^n + z^n=0$ and $xyz \ne 0$, then $$\frac{(x^n-y^n)^2}{(xy)^n} + \frac{(y^n-z^n)^2}{(yz)^n} + \frac{(z^n-x^n)^2}{(zx)^n} = -9.$$

share|cite|improve this answer
    
Very cool. Is there a short way to prove this? – DanielV Jun 25 '14 at 3:02
    
@DanielV: I found this in my sketchbook from February 2008, but unfortunately there was no derivation included. However, it's fairly easy to derive in any number of ways. Here's one: \begin{align} 0^3 &= (x^n+y^n+z^n)^3 \\ &= x^{3n}+y^{3n}+z^{3n} + 3(x^n+y^n)(y^n+z^n)(z^n+x^n) \\ &= x^{3n}+y^{3n}+z^{3n} - 3(xyz)^n \\ 3(xyz)^n &= x^n(-x^n)^2 + y^n(-y^n)^2 + z^n(-z^n)^2 \\ &= x^n(y^n+z^n)^2 + y^n(z^n+x^n)^2 + z^n(x^n+y^n)^2 \\ &= x^n(y^n-z^n)^2 + y^n(z^n-x^n)^2 + z^n(x^n-y^n)^2 + 12(xyz)^n, \end{align} and since $xyz\ne0$ (by standard FLT hypothesis), the identity quickly follows. – Kieren MacMillan Jun 26 '14 at 12:22
2  
A related — and similarly surprising — identity is $$\biggl(\frac{x^2}{yz}\biggr)^{\!n} + \biggl(\frac{y^2}{xz}\biggr)^{\!n} + \biggl(\frac{z^2}{xy}\biggr)^{\!n} = 3.$$ – Kieren MacMillan Jun 26 '14 at 22:56

Another surprising equation $$\bbox[7pt,border:3px #FF69B4 solid]{\color{red}{\large 213 \times 122 = 25986}}$$ Now read above expression in reverse order $$\bbox[7pt,border:3px #FF69B4 solid]{\color{red}{\large 68952 = 221 \times 312}}$$

share|cite|improve this answer
1  
Here is another one $$\bbox[7pt,border:3px #FF69B4 solid]{\color{red}{\large 221 \times 113 = 24973}}$$ Now read above expression in reverse order $$\bbox[7pt,border:3px #FF69B4 solid]{\color{red}{\large 37942 = 311 \times 122}}$$ – Venus Jan 7 '15 at 19:01

$$\int_0^1\frac{x^4(1-x)^4}{1+x^2}=\frac{22}{7}-\pi$$

It's interesting how something so bizarre on the left hand side yields the tiniest of errors in one of the most famous approximations of $\pi$.

share|cite|improve this answer

It is still strange for me $$ i^i = e^{\pi(2k-\frac{1}{2})}. $$ And so, one could say $i^i\in\mathbb R$.


Note that $i^i$ is a sequence of real numbers and actually $i^i\not\in\mathbb R$, but still $i^i\subset\mathbb R$.

share|cite|improve this answer

Personally I find this very interesting: $$ \lim_{n\to\infty} e^{-n}\sum_{k=0}^n \frac{n^k}{k!}=\frac{1}{2}. $$

share|cite|improve this answer
    
Could you please include a proof? – Potato Sep 26 '13 at 22:55
1  
    
Thanks. $\textbf{}$ – Potato Sep 26 '13 at 23:35
    
Also: math.stackexchange.com/questions/160248/… – Arash Oct 4 '13 at 11:06

$$\cos \left(20\right) \cos \left(40\right) \cos \left(80\right) = \frac{1}{8}$$ for angles in degrees. This identity is interesting for its historical association with the teenage Richard Feynman. From Genius by James Gleick:

"He and his friends traded mathematical tidbits like baseball cards. If a boy named Morrie Jacobs told him that the cosine of 20 degrees multiplied by the cosine of 40 degrees multiplied by the cosine of 80 degrees equaled exactly one-eighth, he would remember that curiosity for the rest of his life, and he would remember that he was standing in Morrie's father's leather shop when he heard it."

share|cite|improve this answer
3  
Hint: Generalisation: $\cos(60-x)\cos(60+x)\cos(x)=\cos(3x)/4$. It gets even more interesting with tangents: $\tan(60-x)\tan(60+x)\tan(x)=\tan(3x)$ – chubakueno Sep 29 '13 at 18:51
    
Also $\sin(60-x)\sin(60+x)\sin(x)=\sin(3x)/4$ (just multiple both your $\cos$ and $\tan$ identities). – user26486 Apr 3 '15 at 13:09

Using the mystical ennead to calculate the decimal expressions for fractions of 7. Start with this figure:

numbered ennead

Then follow the connected path, giving the sequence 1 4 2 8 5 7. Then you write this sequence starting on each digit, in order, giving

. 1 4 2 8 5 7 = 1/7

. 2 8 5 7 1 4 = 2/7

. 4 2 8 5 7 1 = 3/7

. 5 7 1 4 2 8 = 4/7

. 7 1 4 2 8 5 = 5/7

. 8 5 7 1 4 2 = 6/7

share|cite|improve this answer
    

I've found this to be rather surprising:

  • $\displaystyle\sum\limits_{n=1}^{\infty}\frac{1}{2^n}=1$

  • $\displaystyle\sum\limits_{n=1}^{\infty}\frac{1}{2^n\ln(2^n)}=1$

As it essentially yields the identity:

$$\sum\limits_{n=1}^{\infty}\frac{1}{2^n}=\sum\limits_{n=1}^{\infty}\frac{1}{2^n\ln(2^n)}$$

It is surprising because obviously:

$$\forall{n\in\mathbb{N}}:\frac{1}{2^n}\neq\frac{1}{2^n\ln(2^n)}$$

In fact, the above inequity holds for every value of $n$, except for $n=\log_2e$.

Still, when summing up each of these infinite sequences, the result is $1$ in both cases.

share|cite|improve this answer

$$\sum_{k=-\infty}^\infty 2^k = 0$$ as can be shown from the regularization $\sum\limits_{k=1}^\infty 2^k=-1$. I'm wondering whether this is not actually the case for all ("sensible") two-sided regularizations, see here


Too explain this sum, note how for $|q|<1$ the geometric series $1+q+q^2+...$ converges to $\frac1{1-q}$. This works fine for the negative powers of two, i.e. $q=\frac12$ such that $\sum\limits_{k=-\infty}^02^k=\sum\limits_{k=0}^\infty\left(\frac12\right)^k=2$. Regularization now basically consists of stating "Ok, outside the convergence region (the positive powers of two in this case) just claim $1+q+q^2+...$ is still "equal" to $\frac1{1-q}$", i.e. $\sum\limits_{k=0}^\infty 2^k "=" \frac1{1-2}=-1$. Subtract the $1=2^0$ counted twice from these two sums to get above result.

We Theoretical Physicists tend to do things like this regularly, which mostly means we were too eager on swapping $\lim$s at some point before ;)

share|cite|improve this answer
3  
How can the sum from 1 to infinity of positive integers lead to a negative number? – Mew Sep 28 '13 at 2:27
    
@Chris Because it isn't a sum in the normal sense. It's a regularized divergence. – Potato Sep 28 '13 at 5:48
1  
@Chris Sorry for the brevity, I expanded a bit on this – Tobias Kienzler Sep 28 '13 at 7:22
1  
@Chris You need to review the "allowable" operations on summations. SumToInfinity(F(X)) is not the same as F(0) + F(1) + F(2) + ..., otherwise you'd have paradoxes even in convergent sums due to the fact that you can arbitrary choose the order of which terms to sum. – DanielV Sep 28 '13 at 14:11
2  
The Casimir effect needs a trick like that. – Felix Marin Sep 29 '13 at 9:52

$$(1+2+3+\cdots+n)!=1!3!5!\cdots(2n-1)!$$for $n=0,1,2,3,4$.

share|cite|improve this answer

This is the most surprising result that I am the discoverer of.

Consider the diophantine equation $$x(x+1)...(x+n-1) -y^n = k$$

where $x, y, n,$ and $k$ are integers, $x \ge 1$, $y \ge 1$, and $n \ge 3$.

I was led to consider considering this by trying to generalize the Erdos-Selfridge result that the product of consecutive integers could never be a power.

I phrased this as "How close and how often can the product of $n$ consecutive integers be to an $n$-th power?"

Looking at this equation, it seemed reasonable to think that, for fixed $k$ and $n$, there were only a finite number of $x$ and $y$ that satisfied it. This was not too hard to prove.

What greatly surprised me was that I was able to prove that for any fixed $k$, there were only a finite number of $n$, $x$, and $y$ that satisfied it.

The proof went like this:

I first showed that any solution must have $y \le |k|$. This was moderately straightforward, and involved considering the three cases $y < x$, $x \le y \le x+n-1$, and $y \ge x+n$.

The next step really surprised me. I showed that $n < e|k|$, where $e$ is the good old base of natural logarithms.

The proof was amazingly (to me) simple. Since $y \le |k|$ and $2(n/e)^n < n!$,

$\begin{align} 2(n/e)^n &< n!\\ &\le x(x+1)...(x+n-1)\\ &= y^n+k\\ &\le |k|^n+|k|\\ &\le |k|^n+|k|^n\\ &= 2|k|^n\\ \end{align} $

so $n < e |k|$.

I still remember staring at this in disbelief, over forty years later.

share|cite|improve this answer
1  
+1 Interesting. BTW, didn't you mean to write "the Erdos-Selfridge result"? – r.e.s. Oct 3 '13 at 3:23
    
You are right, of course. Thanks. I will fix. – marty cohen Oct 5 '13 at 22:38
    
Oh nice, didn't expect it to be finite. – Calvin Lin Oct 10 '13 at 0:34

I have always been fascinated by Leibniz's formula for $\pi$: $$\dfrac{\pi}{4}=\sum\limits_{n=0}^\infty \dfrac{1}{2n+1}\times(-1)^{n}=1-\dfrac{1}{3}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{9}-\dfrac{1}{11}\dots$$ This can be used to determine the exact value of $\pi$, which is what makes it interesting. $$\displaystyle \boxed{\pi=4\sum\limits_{n=0}^\infty \dfrac{1}{2n+1}\times(-1)^{n}=4-\dfrac{4}{3}+\dfrac{4}{5}-\dfrac{4}{7}+\dfrac{4}{9}-\dfrac{4}{11}\dots}$$

share|cite|improve this answer
    
I would classify this as a "'standard' / well-known result," but it's cool nonetheless – Michael Tong Feb 28 '14 at 2:29
    
Unfortunately, this formula converges pathetically slowly to be of any use =( – Trogdor Sep 7 '15 at 17:06

This one is no less- Let $d$ be the distance between Incenter($r$) and Circumcenter ($R$) Then-
$$R^2-d^2=2Rr$$ and this one $$\frac{1}{R-d}+ \frac{1}{R+d}=\frac{1}{r}$$

share|cite|improve this answer
1  
Thanks. Those do seem strange, till you see how it is set up. – Calvin Lin Sep 27 '13 at 13:11

I know it's incredibly simple, but I'm always awed by $$ 2+2 = 2 \cdot 2 = 2^2 = \;^2 2. $$ Two is where addition, multiplication and exponentiation meet. And: tetration.

share|cite|improve this answer
23  
That extends beyond exponentiation, as well, in the sense that 2 op 2 has the same value for every binary operation op in Goodstein's infinite sequence of hyperoperations (+, *, ↑, ↑↑, ↑↑↑, ...). – r.e.s. Sep 27 '13 at 4:23
1  
@r.e.s. Can you explain that? Isn't 2 ↑↑ 2 = 16 ? – MrZander Sep 28 '13 at 0:04
3  
@MrZander For any op beyond addition, in the expression x op y the y specifies how many x's are to be "combined" using the hyperoperator at the next lower level. E.g., 2↑↑4 = 2↑2↑2↑2 = 2↑2↑4 = 2↑16 = 65536, 2↑↑3 = 2↑2↑2 = 2↑4 = 16, 2↑↑2 = 2↑2 = 4. – r.e.s. Sep 28 '13 at 3:57

Let $p_n$ be the probability that a random permutation in the symmetric group $S_n$ doesn't have fixed points. Then $\lim_{n\to\infty}p_n=\frac{1}{e}$.

I was amazed the first time I saw this exercise!

share|cite|improve this answer
    
No big surprise. This is just an application of the inclusion-exclusion formula and $\displaystyle{e^x=\sum_{n\geq0}\frac{x^n}{n!}}$ – Taladris Oct 19 '13 at 12:44
1  
Of course that's how it's proven, but it's still surprising when you first see it! Many of the facts on this post are easy and not surprising once you know what's going on behind the scenes! – rfauffar Oct 19 '13 at 18:14

from $1$ years ago

$$\tan x=\cfrac{x}{1-\cfrac{x^2}{3-\cfrac{x^2}{5-\cfrac{x^2}{7-...}}}}$$

share|cite|improve this answer

$$\sum_{i=0}^N {{N}\choose{i}}=2^N$$

share|cite|improve this answer

Given a polynomial $p(x)$ of degree $n$, let $a$ be the leading coefficient. Then:

$$\sum_{k=0}^n (-1)^k{n\choose k}p(x-k)=an!$$

This happens to be equivalent to:

$$p^{(n)}(x)=an!$$

where $p^{(k)}$ is the $k$th derivative of $p(x)$.

The surprising part is that the sum can actually obtain the leading coefficient without any remaining reference to the polynomial aside from the factorial of the degree.

share|cite|improve this answer
    
An instance of the binomial transform which, together with its inversion, form a nice couple of formulas. – Jean-Claude Arbaut Oct 26 '15 at 17:39

The infinite Fibonacci sequence $F = (0, 1, 1, 2, 3, 5, 8, \dots)$ and the infinite Fibonacci string $S = 1011010110110 \dots$ are related by the following remarkable identity for all real or complex $\beta$ such that $\ |\beta| > 1$:

$$[0 \ ; \ \beta^{F_0}, \beta^{F_1}, \beta^{F_2}, \dots] = (\beta - 1) \cdot (0.S)_\beta$$

where $[0 \ ; \ \beta^{F_0}, \beta^{F_1}, \beta^{F_2}, \dots]$ denotes the continued fraction

$$\frac{1}{\beta^{F_0} + \frac{1}{\beta^{F_1} + \frac{1}{\beta^{F_2} + \cdots}}} $$

and $(0.S)_\beta = (0.1011010110110 \dots)_\beta$ denotes the number obtained by reading $0.S$ as a "base-$\beta$ numeral"; that is, $(0.S)_\beta$ denotes the sum of the infinite series

$$S[1] \beta^{-1} + S[2]\beta^{-2} + S[3]\beta^{-3} + \cdots$$

where $S[n]$ is the $n$th element of string $S$.

E.g., the so-called rabbit constant $(0.S)_2$ = 0.709803... in decimal , $(0.S)_\pi$ = 0.362011... in decimal, etc.

($F$ and $S$ are also related by the fact that both are generated by recursions of the form $x_{n+1} = x_{n} + x_{n-1} ;\ x_0 = 0, \ x_1 = 1$, where the $+$ is interpreted in one case as arithmetic addition, and in the other case as string concatenation.)

share|cite|improve this answer

$$(1+i)(1+2i)(1+3i) = (1-i)(1-2i)(1-3i)$$

share|cite|improve this answer
    
Interesting, it's actually correct! – DanielV Nov 22 '14 at 17:53
    
Ah , I get it, this is a consequence of the $$\begin{align}\arctan(1) + \arctan(2) + \arctan(3) = \pi \text{ radians } \\ = -\pi \text{ radians } = \arctan(-1) + \arctan(-2) + \arctan(-3)\end{align}$$ – DanielV Nov 22 '14 at 17:59

protected by Zev Chonoles Sep 27 '13 at 7:46

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.