# A problem about the largest prime factor of $n^2+1$

Let $f(n)$ be the largest prime factor of $n$. The image of function $g(n)=\sqrt{f(n^2+1)}$ is like this:

Question: If we want to draw a horizontal line which bisects the points from $n=1$ to $n=x,$ this line is probably in what position when $x\to \infty$?

The first thing we can find on the image is that there are many approximate straight lines.

Then what's the slope of these straight lines?

I find that the slope of the most inclined straight line is $1,$ hence the equation is $y=x.$ The second line is $y=\dfrac{x}{\sqrt2},$ the third is $y=\dfrac{x}{\sqrt5},$ then $y=\dfrac{x}{\sqrt{10}},$ then $y=\dfrac{x}{\sqrt{13}}.$

$\dfrac{1}{\sqrt{m}}$ can be a slope if and only if $a^2+1\equiv 0\pmod m$ has integer solution.

In fact, the point $(n,\sqrt{f(n^2+1)})$ on $y=x$ means $g(n)\approx n$ hence $n^2+1$ is prime. $y=\dfrac{x}{\sqrt2}$ means $g(n)\approx \dfrac{n}{\sqrt2}$ hence $2\mid n^2+1$ and $\dfrac{n^2+1}{2}$ is prime $\dots$

Whether there are infinity many primes of the form $n^2+1$ is still an open problem. So we can not prove these conclusions, but we can estimate it.

• I think you are asking for $c=c(x)$ such that the number of $n\le x$ with $g(n)\lt c$ is $n/2$. Have you tried tabulating $c(x)$ for $1\lt x\le10000$ to see what it looks like? Jun 25, 2013 at 12:51

## 2 Answers

What you are trying to calculate is the median largest prime factor of $$f(n)=n^2+1$$, where $$1\leq n\leq x$$. This will be a function of $$x$$, which we denote by $$M_{n^2+1}(x)$$, and this will grow to infinity as $$x$$ grows to infinity.

To get a handle on things, lets first take a step back, and ask an easier question:

What is the median largest prime factor of the integers $$n$$ in the range $$1\leq n\leq x$$?

Let us denote this function by $$M(x)$$. This question was raised on Math Overflow, and I proved that we have the asymptotic $$M(x)\sim e^{\frac{\gamma-1}{\sqrt{e}}}x^{\frac{1}{\sqrt{e}}},\ \ \ \ \ \ \ \ \ \ (1)$$ where $$\gamma$$ is the Euler Mascheroni constant. See the corresponding paper on the arXiv for details.

So what about $$n^2+1$$? This seems like it must be significantly more difficult, as now we are dealing with the prime factors of a polynomial, which usually increases the complexity of a problem significantly. Modifying section $$4$$ of the paper I mentioned above, we can relate the problem to $$\psi_{f}(x,y)=\left|\{n\leq x:\ P(f(n))\leq y\}\right|$$ when $$f(n)=n^2+1$$, where $$P(n)$$ denotes the largest prime factor of $$n$$. Unfortunately, not too much is known about $$\psi_f(x,y)$$ when $$f$$ is a polynomial of degree $$2$$ or higher. The specific case $$f(n)=n^2+1$$ has been looked at by Dartyge, and Harman, where they give lower bounds on the order of magnitude for $$u=\frac{\log x}{\log y}$$ in a fairly small range. To obtain an asymptotic however, we would need something stronger - a precise asymptotic for $$\psi_{n^2+1}(x,y)$$, where $$f(n)=n^2+1$$, with error of size $$\frac{x}{\log^2 x}$$. (That is, we need to know the $$\frac{x}{\log x}$$ term as well) Under a major assumption, Martin proves that we do in fact have $$\psi_F(x,y)\sim x\rho\left(\frac{\log F(x)}{\log y}\right)$$ when $$F$$ is an irreducible polynomial, and where $$\rho$$ is the Dickman de Bruijn rho function. ($$u=\frac{\log x}{\log y}$$ will lie in a particular bounded range) Using Martin's result, and the methods I mentioned above, we are able to obtain that your quantity is $$M_{n^2+1}(x)\approx x^{2/\sqrt{e}}.$$ Note that this requires assuming a version of Hardy and Littlewood's second conjecture.

Personally, I am not entirely satisfied without an asymptotic. I believe that Martin's work can be modified to show that $$\psi_F(x,y)=x\Lambda\left(F(x),y\right)+O_F\left(\frac{x}{\log^2 x}\right)$$ for $$u=\frac{\log x}{\log y}$$ in a particular bounded range, and where $$\Lambda(x,y)$$ is a function appearing in de Bruijn's original paper on the subject. Using the expansion for $$\Lambda(x,y)$$ appearing in Eric Saias' 1989 paper, we would be able to recover the asymptotic

$$M_{n^2+1}(x) \sim e^{\frac{\gamma-1}{\sqrt{e}}} x^{2/\sqrt{e}}, \ \ \ \ \ \ \ \ \ (2)$$ which is very similar to equation $$(1)$$ above.

Moreover, I believe that given any irreducible integer polynomial $$F$$ of degree $$d$$, we have that the median largest prime factor of $$F(n)$$ in the interval $$[1,x]$$ satisfies the asymptotic $$M_F(x)\sim e^{\frac{\gamma-1}{\sqrt{e}}} x^{d/\sqrt{e}}.\ \ \ \ \ \ \ \ \ (3)$$

Under the assumptions in Martin's paper, the above outline should be able to prove that equation $$(3)$$ holds when the degree is less than or equal to $$2$$. However, additional issues arise when $$d\geq 3$$. Specifically, there are problems regarding the range of $$u$$ which I did not discuss so thoroughly above, and the previous bounds we had on $$\psi_F(x,y)$$ are not in a large enough range of $$u$$ for the proof to work.

I hope that answers your question, and that equation $$(2)$$ is what you are looking for.

Here's some non-optimized code

prim = Table[First[Last[FactorInteger[n^2 + 1]]], {n, 1, 100000}];
ListPlot[Table[With[{temp = Sort[Take[prim, 2 k]]}, (temp[[k]] + temp[[k + 1]])/2],
{k, 2, 20000}]]


That leads to the following picture. I'd say that's too irregular to predict long-term behavior.

EDIT -- I was multiplying by 2, twice, which skewed the values. Here's better code.

prim = Table[First[Last[FactorInteger[n^2 + 1]]], {n, 1, 100000}];
sortedprimes = {};
vals = {};
Monitor[
Do[
addprime = Sort[{prim[[k - 1]], prim[[k]]}];
sortedprimes = Sort[Join[sortedprimes, addprime]];
newval = (sortedprimes[[k/2]] + sortedprimes[[k/2 + 1]])/2;
vals = Append[vals, newval],
{k, 2, 100000, 2}],
k]
ListPlot[Transpose[{Range[50000] 2, vals}]]


Gives the following picture

We can try fitting the data in various ways. Fitting to $A x + B x/log(x)$ didn't immediately go bad when more terms were added.

ListPlot[{#[[1]], #[[2]] - (25.1 #[[1]] - (185.1 #[[1]])/
Log[#[[1]]])} & /@ Transpose[{Range[50000] 2, vals}]]


Leading to the picture

• I am not sure if your data is accurate. The median largest prime factor of $n^2+1$ on the interval $[1,10000]$ is $55287$, whereas your graph suggests that it is close to $150 000$. Jun 27, 2013 at 0:37
• I corrected the code. Jun 27, 2013 at 4:18
• Looks good now. Try best fitting with the function I mention in my answer, $$e^{(\gamma-1)/\sqrt{e}}\cdot x^{2/\sqrt{e}}.\ \ \ \ \ \ \ \ (2)$$ Numerically, this is $$\approx 0.773808\cdot x^{1.21306}.$$ The error between this and the true value should be strictly less than the main term multiplied by $\frac{1}{\log x}$. At $x=10000$, the true value is $55287$, and the estimate above, equation $(2)$, gives $55065$, which is an error of $222$. Jun 27, 2013 at 19:23