77
votes

### Is there any deep philosophy or intuition behind the similarity between $\pi/4$ and $e^{-\gamma}$?

The intuition may be helped by considering the generalized Euler constant function
$$\gamma(z)=\sum_{n=1}^\infty z^{n-1}\left(\frac{1}{n}-\ln\frac{n+1}{n}\right),\;\;|z|\leq 1.$$
Its values include ...

33
votes

Accepted

### Is the integral of $e^{-x^2}$ from $0$ to $1$ known to be irrational?

Following @te4's comment, we can look at the function $$f(x) = \frac{\sqrt{\pi}}{2\sqrt x} \operatorname{erf}(\sqrt{x}) = \sum_{n=0}^\infty \frac{(-1)^n x^n}{(2n+1) n!}.$$ Note that it's an E-function,...

27
votes

Accepted

24
votes

Accepted

### Representations of $\zeta(3)$ as continued fractions involving cubic polynomials

See NOTE below.
This MO inquiry is over 3 yrs old now.
By the date the question about the $\zeta(3)$ CF with $k=8/7$ was made (Feb, 2019), it can be answered in the negative nowadays, since it was '(...

22
votes

### A new way of approaching the pole of the Riemann zeta function - and a new conjectured formula

The paper entitled Euler constant as a renormalized value of Riemann zeta function at its pole by Andrei Vieru contains a derivation of the first formula in the OP (Benoȋt Cloitre's formula), and ...

22
votes

### Closed form of an infinite series

Denote $c_n:={(-1)^n \frac{\Gamma(\frac{1}{3}+\frac{n}{3})}{\Gamma(1+\frac{n}{3})} \sin(\frac{2\pi n}{3})}$ the $n$-th term of the series. We have for all $k\ge0$
$$c_{3k}=0,$$
$$c_{3k+1}= (-1)^{k+1}\...

21
votes

### Is the integral of $e^{-x^2}$ from $0$ to $1$ known to be irrational?

$\newcommand{\Q}{\mathbb Q}\newcommand{\erf}{\operatorname{erf}}$(This answer had been posted before I saw Command Master's answer. I am leaving it here, since it contains more and/or different ...

18
votes

### Power series $x^n/n$ with one plus, then two minuses, then three plusses, and so on

The answer is in terms of theta functions. Let $$\Psi(q)=1+q+q^3+q^6+q^{10}+q^{15}+\cdots;$$ this is a Jacobi theta function that can be written as $q^{-1/8}\eta^2(2\tau)/\eta(\tau)$ where $q=e^{2\pi ...

17
votes

Accepted

### Any name for this special function?

This is a standard hypergeometric function. Note that
$$ \frac{1}{(a-m)!} = (-1)^m \frac{(-a)_m}{a!}\quad\text{and}\quad \frac{1}{(b+m)!} = \frac{1}{b!\,(b+1)_m}$$
in terms of the rising Pochhammer ...

16
votes

Accepted

### are these polynomials or rationals functions?

This is response to QUESTION 1.
As Fedor pointed out, we're dealing with the Chebyshev polynomials $P_n(2\cos t)=\sin nt/\sin t$. So we must show that if
$$
\sum_{n=1}^N \sin nt = 0 , \quad\quad\quad\...

16
votes

### A new way of approaching the pole of the Riemann zeta function - and a new conjectured formula

The first formula is trivial. $$f(s)= \frac1{s-1}+\gamma +O(s-1)$$ $$g(z)=1+2^{-z}+3^{-z}+4^{-z}+O(5^{-z})=1+2^{-z}(1+(3/2)^{-z}+(4/2)^{-z}+O(5/2)^{-z})$$
$$f(g(z)) = \frac1{2^{-z}(1+(3/2)^{-z}+(4/2)^...

16
votes

Accepted

### Is there a specific named function that is the inverse of $x+x^a$ for $x$ real?

The answer is yes indeed. It is a special case of Fox-H function, a variation of the confluent Fox-Wright $_{1}\Psi_{1}$ function (a generalization of the confluent hypergeometric function $_{1}F_{1}$)...

15
votes

### How to prove Lambert's W function is not elementary?

In [Ritt 1948] p. 53 - 56, the method of J. Liouville is given for Kepler's equation. The same method can be applied to functions $f$ with $f(z)=A(z,e^z)$ ($A$ an algebraic function of two complex ...

15
votes

Accepted

### Short research articles

[a bit too long for a comment]
I understand from the question that the aim is to find a research project based on the search for a counterexample. By construction, this will mean showing that some ...

Community wiki

15
votes

Accepted

### Question about functions $f: \mathbb{Z}^+ \to \mathbb{Z}^+$ such that $x$ is prime whenever $f(x)$ is prime

As observed in comments, we have $f(n) = \lfloor g(n) \rfloor$ where $g(n) = \frac{\alpha^n + \alpha^{-n}}{4}$ and $\alpha = 2 + \sqrt{3}$. From the recurrence $g(n+1) = 4 g(n) - g(n-1)$ we see that $...

14
votes

Accepted

### Series involving factorials

The sum
$$\sum_{k=0}^\infty \frac{(a+k)!\,(b+k)!}{k!\,(a+b+c+k+1)!}z^k.$$
is not only a generalized hypergeometric series; it's the original ungeneralized Gauss hypergeometric series,
$$\frac{\Gamma(a+...

14
votes

Accepted

### A surprising identity: $\det[\cos\pi\frac{jk}n]_{1\le j,k\le n}=(-1)^{\lfloor\frac{n+1}2\rfloor}(n/2)^{(n-1)/2}$

First of all, we use the formula
$$
D:=\det [x_j^k+x_j^{-k}]_{j,k=0,\dots,m-1}=\prod_{l<j}(x_j+x_j^{-1}-x_l-x_l^{-1})=\prod_{l<j} (x_j-x_l)(1-x_j^{-1}x_l^{-1}).
$$
This follows from the ...

14
votes

### A new formula for the class number of the quadratic field $\mathbb Q(\sqrt{(-1)^{(p-1)/2}p})$?

If by
"the class number $h(p^*)$ of the quadratic field $\mathbb{Q}(\sqrt{p^*})$"
you mean
"the minus class number $h^{-}$ of $\mathbf{Q}(\zeta_p)$"
and if by
" a possible new formula for the ...

13
votes

Accepted

### Summation of series involving $\sinh$ of a square root

Here is a solution that I have found while working on other lattice sums.
It utilizes a very simple result:
Define $f$ by
$$f(x)=\sum_{n=0}^{\infty} (-1)^n (2n+1) e^{-\pi x (n+\frac12)^2}.$$
...

13
votes

Accepted

### Rotation invariance of an integral

Yes. Denote $p=\sinh s$, then $s\in (-\infty,\infty)$, $\sqrt{p^2+1}=\cosh s$, $dp=\cosh s\, ds$. Next, denote $r=\sqrt{\tau^2+x^2}$, $\tau=r\cos \varphi$, $x=r\sin \varphi$, where $\varphi\in (-\pi/2,...

13
votes

### A new formula for the class number of the quadratic field $\mathbb Q(\sqrt{(-1)^{(p-1)/2}p})$?

The conjecture is not true, as some examples show.
Let $D(p)$ denote your number. For primes $p \equiv 1 \textrm{ mod } 4$, we have $D(29)=8$, $D(37)=37$, $D(41)=121$ while $h(29)=h(37)=h(41)=1$.
...

13
votes

Accepted

### A multiple integral that seems related to the $\zeta$ function at even integers

We have $$I_{2n}=\frac{(2n)!!}{(2n+1)!!}\cdot \frac1{2n+2}\cdot \pi^{2n}.$$
To see this, we follow the suggestion by Terry Tao in the comments and apply the diagonalization of the integral operator ...

13
votes

Accepted

### Speed of convergence of $\zeta(2k)\to 1$?

Here is an explicit bound. The sum $\sum_{n > N} n^{-s}$ for real $s > 1$ is bounded by the integral
$$\int_N^\infty x^{-s} = N^{1-s} / (s-1).$$
Therefore for any $N$ you have
$$0 < \zeta(s) -...

13
votes

Accepted

### Riemann, fluid dynamics, and critical lines

Q: Does anyone know of a reference which discusses more thoroughly the critical line appearing in Riemann's hydrodynamics problem?
A: A recent reference is Elliptical instability in hot Jupiter ...

12
votes

Accepted

### Evaluating elliptic integrals

This question has been out for a while, and I think it deserves a thorough answer, even tho I came quite late to this party.
As both the classical Legendre-Jacobi theory and the Carlson theory have ...

12
votes

Accepted

### Asymptotics of a special function

Gradshteyn & Ryzhik equation 3.773.1 gives (for $q>0$)
$$\frac{1}{q}B(q)=\int_0^\infty \frac{\sin x}{x(q^2/4+x^2)^{1/2}}\,dx=\frac{1}{q}G^{21}_{13}\left(\frac{q^2}{16}\biggl|^1_{1/2,1/2,0}\...

12
votes

### A tantalizing Gamma quotient to challenge the Rohrlich-Lang Conjecture

A conceptual way to tackle this question is to look at universal distributions on $\mathbf{Q}/\mathbf{Z}$, studied by Kubert and Lang among others. Distributions arise naturally in number theory, see ...

12
votes

### A variant of Lambert function

Sorry, my answer is wrong. As it was pointed out by "Simply Beautiful Art" $x\ne W(z)$ but $x=e^{W(z)}.$
It is known that $$W'(z)=\frac{W(z)}{z(1+W(z))}.$$
Let $z=\log t$. Then $x=W(z)$ is a root of ...

12
votes

Accepted

### Is the Gauss hypergeometric series ${}_2F_1\bigl(\frac{1}{2},\frac{1}{2};2;t\bigr)$ an elementary function?

Maple does it in terms of complete elliptic integrals $\rm{K}$ and $\rm{E}$ ...
$$
{\mbox{$_2$F$_1$}\left(\frac12,\frac12;\,2;\,t\right)}={\frac {4\left( t-1 \right){\rm K} \left(
\sqrt {t} \right) ...

12
votes

Accepted

### Closed form of an infinite series

Q: Does the following infinite series have a closed form?
It does, according to Mathematica:
$$\sum_{n=1}^{\infty} {(-1)^n \frac{\Gamma(\frac{1}{3}+\frac{n}{3})}{\Gamma(1+\frac{n}{3})} \sin\left(\frac{...

Only top scored, non community-wiki answers of a minimum length are eligible

#### Related Tags

special-functions × 833ca.classical-analysis-and-odes × 187

hypergeometric-functions × 105

real-analysis × 96

integration × 92

nt.number-theory × 88

cv.complex-variables × 80

reference-request × 76

sequences-and-series × 73

asymptotics × 53

co.combinatorics × 49

fa.functional-analysis × 49

bessel-functions × 48

gamma-function × 45

orthogonal-polynomials × 37

modular-forms × 33

polynomials × 32

inequalities × 32

pr.probability × 31

differential-equations × 25

mp.mathematical-physics × 23

closed-form-expressions × 22

integral-transforms × 21

na.numerical-analysis × 18

power-series × 17