For questions on infinite products: convergence, computation, etc...

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1answer
26 views

Is it true that factor spaces are T4 if product space is T4?

I use the following definition of $T_4$-space: for any two disjoint closed sets $A$, $B$ there exist disjoint open sets $U$, $V$ containing $A$ and $B$ respectively. Is it true that factor spaces are ...
2
votes
1answer
29 views

Error estimation for the Wallis product

From the Wallis product we know $$\prod_{k=1}^{\infty} \left(\frac{2k}{2k-1} \cdot \frac{2k}{2k+1}\right) = \frac{2}{1} \cdot \frac{2}{3} \cdot \frac{4}{3} \cdot \frac{4}{5} \cdot \frac{6}{5} \cdot ...
2
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0answers
13 views

What are the statistics of the discrete Fourier transform of a Bernoilli process?

The problem I would like to understand the statistics of the discrete Fourier transform of a sequence of uncorrelated events $\{x_n\}$ each of which takes the value $\pm1$ with probability $1/2$. In ...
1
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0answers
43 views

Evaluating Continued Product of Radicals

I am trying to find the following continued product $y = \sqrt{p}.\sqrt{p+p.\sqrt{p}}. \sqrt{p+p.\sqrt{p+p\sqrt{p}}}...........\infty$ for $p=\frac12$ First I squared the LHS and RHS to get $y^2 = ...
2
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0answers
53 views

Prove that this infinite product converges uniformly,

Let $(a_n)_{n=1}^\infty$ be a sequence of complex numbers such that (i) $0<|a_n|<1$, (ii) $\sum_{n=1}^\infty(1-|a_n|)<\infty$ Prove that the infinite product $$\prod_1^\infty \frac{(a_n ...
3
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1answer
78 views

Can we say $(\prod_{n=1}^{\infty}n)^2=(\prod_{n=1}^{\infty}n^2)$

Can we say $$(\prod_{n=1}^{\infty}n)^2=(\prod_{n=1}^{\infty}n^2)$$ It works fine if things are finite, does it hold in $n$ goes to infinity?
4
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2answers
35 views

How to get to this equality $\prod_{m=1}^{\infty} \frac{m+1}{m}\times\frac{m+x}{m+x+1}=x+1$?

How to get to this equality $$\prod_{m=1}^{\infty} \frac{m+1}{m}\times\frac{m+x}{m+x+1}=x+1?$$ I was studying the Euler Gamma function as it gave at the beginning of its history, and need to ...
10
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1answer
76 views

Asymptotic behaviour of log log sum

I am interested in the asymptotics of $$F(m) := \prod_{j=1}^m \log(j+1) = \exp\left(\sum_{j=1}^m \log \log(j+1) \right)$$ Is there anything known? If not I figure I will need some good bounds on the ...
4
votes
2answers
91 views

If $\prod_{n}(1+a_n)$ converges, does $\sum_{n}\frac{a_n}{1+a_n}$ converge?

I have a sequence of complex numbers $a_1,a_2,...$ such that $a_i \neq -1$. I then have the infinite product $\prod_{n=1}^{\infty}(1+a_n)$ which I know converges to a non zero complex number. I was ...
6
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1answer
96 views

calculate $\prod_1^\infty k^{\frac1{k!}}$

Is it possible to find the point of convergence of $\prod_1^\infty k^{\frac1{k!}}$ $K!=k(k-1)!$. My attempt: If $S_n=\prod_1^\infty k^{\frac1{k!}}$ then $\ln S_n=\sum_1^\infty \frac{\ln k}{k!}< ...
8
votes
2answers
127 views

Proving $~\prod~\frac{\cosh\left(n^2+n+\frac12\right)+i\sinh\left(n+\frac12\right)}{\cosh\left(n^2+n+\frac12\right)-i\sinh\left(n+\frac12\right)}~=~i$

How could we prove that $${\LARGE\prod_{\Large n\ge0}}~\frac{\cosh\left(n^2+n+\dfrac12\right)+i\sinh\left(n+\dfrac12\right)}{\cosh\left(n^2+n+\dfrac12\right)-i\sinh\left(n+\dfrac12\right)}~=~i$$ ...
2
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0answers
29 views

Existence of a Blaschke product with an added boundary condition.

Suppose we have a sequence of distinct complex numbers $\{a_n\}$ such that $a_n\rightarrow 1$ and the $a_n$ satisfy the Blaschke condition $\sum (1-|a_n|)<\infty$. Does there exist a Blaschke ...
13
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5answers
448 views

Closed form for an infinite product

The following fascinating formula appears in the paper "On gamma quotients and infinite products" by M.Chamberland and A.Straub (see page 9): ...
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1answer
59 views

What is the limit of this product? (SOLVED)

What does this limit equal? $$\lim\limits_{k\to\infty}\left(\prod_{n=1}^kn^{2^{k-n}}\right)^{\frac{1}{2^{k-1}-1}}$$ All that I have tried so far is computation and it does seem to converge. I ...
1
vote
1answer
36 views

What is the definition of the absolute convergence of an infinite product $(1+a_1)(1+a_2)(1+a_3)\cdots$?

For an infinite product $(1+a_1)(1+a_2)(1+a_3)\cdots$, whats the definition of convergence and absolute convergence? Why the absolute convergence corresponding to the absolute convergence of sum ...
3
votes
1answer
54 views

Some analogs of the pentagonal number theorem

There are the following analogs of the famous identity $$ \prod_{n\geqslant1}(1-q^n)=\sum_{n\in\mathbb Z}(-1)^nq^{\frac{3n^2-n}2}. $$ Let $v_2(n)$ denote the 2-adic valuation of $n$, that is, the ...
6
votes
1answer
209 views

Infinite Product Representation of $\sin x$

I've recently taken interest in infinite products, and I'm having trouble with a proof I found in this PDF file: "Infinite Products and Elementary Functions": An intermediate step in finding an ...
1
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1answer
34 views

Weierstrass factorization theorem and primality function

I'm interested in application of the Weierstrass factorization theorem to the primality function. Let $np(x)\colon \mathbb N\to \mathbb N$ is a "not-prime" function: $$ np(x) = \begin{cases}1, & ...
13
votes
2answers
834 views

How to find this infinite product

How to find this infinite product ? $$\prod_{n=0}^\infty \left(1-\dfrac{2}{4(2n+1)^2+1}\right)$$ I try to use infinite product of $\cos{x}$ but it doesn't work. Thank you.
3
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3answers
139 views

Limit of a product I

While reviewing old problems in American Mathematical Monthly the following problem was encountered. What are some methods to solving the problem ? Proposed by L. S. Johnston, 1929. Consider ...
5
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1answer
56 views

A product version of Riemann integral

Motivated by Riemann sum in Riemann integral and motivated by relations between infinite series and infinite products we ask: Assume that $f:[0, 1]\to \mathbb{R}$ is a positive function. Assume ...
3
votes
1answer
72 views

A golden trigonometric diophantine equation

After answering this question I reflected on the identity $$\cos\frac{\pi}{5}=\phi\cos\frac{\pi}{3}$$ and thought of looking for all the quadruplets of positive integers $(a,b,c,d)$ satisfying $$\cos ...
21
votes
3answers
525 views

How can I prove $\pi=e^{3/2}\prod_{n=2}^{\infty}e\left(1-\frac{1}{n^2}\right)^{n^2}$?

I am interested about some infinite product representations of $\pi$ and $e$ like this. Last week I found this formula on internet ...
6
votes
1answer
111 views

Convergence of $\prod_{n=1}^{\infty}\left(1-\frac{z^2}{n^2}\right)$

I'm looking at some notes from my previous complex variables course and I need help verifying some things about the convergence of $$ \prod_{n=1}^{\infty}\left(1-\frac{z^2}{n^2}\right) $$ on compact ...
2
votes
3answers
487 views

Change from product to sum

We know that : $$a \times b = \underbrace{a + a + a + ... + a}_{\text{b times}}$$ That's how we convert from a product to a sum. So what happens if we go a little further? That is : ...
0
votes
2answers
65 views

How to find whether this series converges or diverges?

Let's suppose I have been given a series that looks like this: $$\sum_{n=1}^n\frac{1\cdot 3\cdot 5\cdot\cdots\cdot(2n-1)}{2\cdot5\cdot8\cdot\cdots\cdot(3n-1)}$$ What I have been thinking of doing ...
0
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2answers
50 views

product converging to one

I have a question concerning an infinite product. Suppose $x_n$ is a sequence of positive real numbers. My intuition says that $$\lim_n(1-\exp(-x_n))^n=1$$ for any sequence $x_n=n^\alpha$ with $\alpha ...
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0answers
24 views

A sort of modified geometric series

I was wondering if there are any hints on how to manage this series $$ \sum_{i=2}^\infty \prod_{j=1}^{i-1} (1-cj^{\beta-1})=(1-c)+(1-c)(1-c2^{\beta-1})+(1-c)(1-c2^{\beta-1})(1-c3^{\beta-1})... $$ with ...
0
votes
2answers
47 views

How to do multiplication (capital pi) in WolframAlpha?

How do i ask this in WolframAlpha: $$\prod_{i=0}^{i=10} \sin{(i)}$$ I used $\text{product}(...)$ and $\text{multiply}(...)$ or even $\text{multiplication}(...)$ but they don't seem to work. I am ...
5
votes
1answer
195 views

This one weird infinite product can define exponentials in terms of itself. What does it do for other constants?

What is... $$\lim_{\omega \to \infty} \prod_{N=1}^{\omega} {{1+e^{b \cdot c^{-N}}} \over 2}$$ This is similar to my other question. However, there is a constant factor rather than variable in the ...
4
votes
3answers
80 views

Is the product $\prod_{k=1}^\infty \frac{2^k-1}{2^k}$ necessarily $0$?

I have the product $\prod_{k=1}^\infty \frac{2^k-1}{2^k}$. I know that every successive partial product will necessarily be smaller than the last, as we are multiplying always by a number smaller than ...
2
votes
2answers
33 views

Ratio test for infinite products?

Is the ratio test applicable for testing convergence of infinite products? In other words, consider the sequence $(a_i)_{i=1}^\infty$ of non-zero real numbers. Also, consider the product ...
5
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3answers
148 views

An infinite product for $e^{x}$

This is again from MathWorld. Let $ x > 0$ and define a sequence $a_{n}$ recursively as follows $$a_{1} = \frac{1}{x}, a_{n} = n(a_{n - 1} + 1)$$ Show that $$e^{x} = \prod_{n = 1}^{\infty}\left(1 + ...
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0answers
16 views

Prove that $\lim_{R \to \infty} \int_{\gamma_R} \frac{z}{(z+2)^3} \prod_{n=1}^{\infty} \frac{\lambda_n - z}{2+\lambda_n+z} dz = 0$

...where $\gamma_R$ is the semicircle in the half plane $\{z \in \mathbb{C}:\Re(z) > -1\}$ of radius $R$ centered at $-1$, assuming that $\sum_{n=1}^{\infty} 1/\lambda_n < \infty$, ...
3
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0answers
39 views

Partitions of integers, a series for infinite product $(1+q)(1+q^3)(1+q^5)\cdots$

Show that $$ (1+q)(1+q^3)(1+q^5) \cdots = 1+ \sum_{k=1}^\infty \frac{q^{k^2}}{(1-q^2)(1-q^4)(1-q^6) \cdots (1-q^{2k})}.$$ How would one proceed combinatorially. What I know is that the left-hand ...
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0answers
27 views

Uniform convergence of an infinite product of automorphisms of the unit disk.

I need to show that $\Pi \frac{|a_n|}{a_n} \frac{a_n - z}{1-\overline{a_n}z}$ with $\{a_n\} \subset D(0,1)$ and $\Sigma$ $(1 -|a_n|) < \infty$ converges to a holomorphic function in $D(0,1)$ It's ...
1
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1answer
29 views

Show that $(1+z)\Pi^{\infty}(1+z^{2^{n}})=\frac{1}{1-z}$ for $z \in \mathbb{C}$ $|z|<1$

$(1+z)\Pi^{\infty}(1+z^{2^{n}})=\frac{1}{1-z}$ for $z \in \mathbb{C}$ $|z|<1$. Any tricks to prove this? I'm not really sure how to start.
2
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2answers
132 views

Let $a_{2n-1}=-1/\sqrt{n}$ for $n=1,2,\dots$ Show that $\prod (1+a_n)$ converges but that $\sum a_n$ diverges.

Let $a_{2n-1}=-1/\sqrt{n}$, $a_{2n}=1/\sqrt{n}+1/n$ for $n=1,2,\dots$ Show that $\prod (1+a_n)$ converges but that $\sum a_n$ diverges. What I have found so far is that $\prod_{k=2}^{2n} ...
4
votes
3answers
139 views

how to evaluate the product $\prod _{n=2}^\infty (1+ \frac{1}{n^2}+\frac{1}{n^4}+\frac{1}{n^6}+\cdots )$? [closed]

Evaluating the infinite product of $\prod _{n=2}^\infty (1+ \frac{1}{n^2}+\frac{1}{n^4}+\frac{1}{n^6}+\cdots )$. Please Help.
0
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0answers
45 views

nth partial product of a double product

I'm trying to find the nth partial product of: $$ \frac{5}{4}^{1/5} \prod_{m=1}^{\infty}\prod_{n=5^{m-1}}^{5^m-1}\left(\frac{5n}{5n+1}\frac{5(n+1)}{5(n+1)-1}\right)^{1/5^{m+1}} $$ I've tried ...
4
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0answers
40 views

How do I find the finite limits of this infinite product?

What is... $$\lim_{\omega \to \infty} \left( {1 \over {a^{\omega}}} \cdot \prod_{N=1}^{\omega} (1+e^{b \cdot c^{-N}}) \right)$$ I'd like closed form solutions, and in this case that means any ...
3
votes
1answer
50 views

$\sum_{n=0}^\infty z^n = \prod_{m=0}^\infty \left(1+z^{2^m}\right)$

When reading Iwaniec and Kowalski's Analytic Number Theory, I came across the following "identity" on page 11 (the Amazon link has a free book preview which includes page 11): $$\sum_{n=0}^\infty z^n ...
1
vote
1answer
46 views

Evaluation of $\prod_{k=1}^{\infty}\frac{a+k^2}{b+k^2}$

While playing around with the question The convergence of a sequence with infinite products, I found Mathematica to give me the result $$ \prod\limits_{k=1}^{\infty}\frac{a+k^2}{b+k^2} = ...
3
votes
2answers
84 views

Compute the values of two infinite products whose factors are the same

I have the following question: How to prove that $(1-\frac{1}{2})\cdot (1+\frac{1}{3})\cdot (1-\frac{1}{4})\cdot (1+\frac{1}{5})\cdot (1-\frac{1}{6})\cdot (1+\frac{1}{7})\cdot ...
0
votes
1answer
54 views

Factorization $\cos(z) - \sin(z)$

How do I find the product expansion of $\cos z - \sin z$ We have $\cos z = \sin z$ iff $z = \pi/4 + k \pi$ where $k$ is an integer. The sequence $\sum (r/(|\pi/4 + k \pi|)^2$ converges For some ...
7
votes
1answer
189 views

What is the limit of this divergent infinite product multiplied by an exponential?

What is... $$\lim_{\omega \to \infty} \left( {1 \over {c^{\omega}}} \cdot \prod_{N=1}^{\omega} (1+e^{b \cdot c^{-N}}) \right)$$ My attempt: I have absolutely no clue except for the case of $c=2$ ...
4
votes
0answers
66 views

Is there anything known about the value where the Euler and Hadamard products for $\zeta(s)$ are equal?

Take the Hadamard product for the Riemann $\xi$-function ($\rho$ is a non-trivial zero of $\zeta(s)$): $$\xi(s) =\frac12\, s\,(s-1) \,\pi^{-\frac{s}{2}}\, \Gamma\left(\frac{s}{2}\right)\, \zeta(s) ...
5
votes
1answer
76 views

Solving $y=\prod_{n=1}^{\infty}\frac{d^ny}{dx^n}$

There is the trivial $y=0$, but beyond that, could there be further solutions for $y$ in terms of $x$ such that $$y=\prod_{n=1}^{\infty}\frac{d^ny}{dx^n}\mbox{ pointwise}$$ ? I posed this problem ...
0
votes
1answer
65 views

Maximise $y$ with respect to $x$ for $y=\prod_{k=1}^{\infty}(1-x^{-k})$

$$y=\prod_{k=1}^{\infty}(1-x^{-k})$$ I want to maximise this function. So far I have: $$\ln(y)=\sum_{k=1}^{\infty}\ln(1-x^{-k})$$ ...
0
votes
1answer
37 views

convergence of an infinite product (how to prove ?)

Fix $C > 0 $ a constant and fix $n \in N$. Consider $\alpha \in (0,1)$ fixed. I am reading a paper and the authors says: For an arbitrary $r <1$ , the infinite product $$P = ...