Questions tagged [sequences-and-series]

For questions concerning sequences and series. Typical questions concern, but are not limited to: recurrence relations, convergence tests, identifying sequences, identifying terms. For questions on finite sums, use the (summation) tag instead.

67
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1k views

Sorting of prime gaps

Let $g_i $ be the $i^{th}$ prime gap $p_{i+1}-p_i.$ If we re-arrange the sequence $ (g_{n,i})_{i=1}^n$ so that for any finite $n$ the gaps are arranged from smallest to largest we have a new sequence ...
58
votes
0answers
1k views

Arithmetic-geometric mean of 3 numbers

The arithmetic-geometric mean$^{[1]}$$\!^{[2]}$ of 2 numbers $a$ and $b$ is denoted $\operatorname{AGM}(a,b)$ and defined as follows: $$\text{Let}\quad a_0=a,\quad b_0=b,\quad a_{n+1}=\frac{a_n+b_n}2,...
34
votes
0answers
611 views

Convergence of $\sum_{n=1}^{\infty} \frac{\sin(n!)}{n}$

Is there a way to assess the convergence of the following series? $$\sum_{n=1}^{\infty} \frac{\sin(n!)}{n}$$ From numerical estimations it seems to be convergent but I don't know how to prove it.
22
votes
0answers
510 views

Are there infinitely many primes of the form $12345678901234567890\dots$

Related to this question, What is the smallest prime number made of sequential number? are there infinitely many primes of the following form (OEIS A057137)? $1, 12, 123, 1234, 12345, 123456, ...
21
votes
0answers
654 views

A Ramanujan-like summation: is it correct? Is it extensible?

I'm still exercising with summation-procedures which I try to make correct Ramanujan-summations. I'm looking at the (gap-)series $$ s(1/2,2) = (1/2)^1+(1/2)^{4}+(1/2)^{9}+(1/2)^{16}+(1/2)^{25}+... $$ ...
20
votes
0answers
499 views

Have I discovered an analytic function allowing quick factorization?

So I have this apparently smooth, parametrized function: The function has a single parameter $ m $ and approaches infinity at every $x$ that divides $m$. It is then defined for real $x$ apart from ...
16
votes
0answers
261 views

Showing that $a_{n+1}=\frac{n}{a_n}-a_n-a_{n-1}$ with $a_0 = 0$ and $a_1=2\Gamma(\frac34)\big/\Gamma(\frac14)$ stays positive for $n\geq1$.

This posting consists of several mildly-related questions, motivated from this posting. The main object is the following sequence. $$a_0 = 0, \qquad a_1 = x, \qquad a_{n+1} = \frac{n}{a_n} - a_n - a_{...
16
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0answers
370 views

Convergence acceleration technique for $\zeta(4)$ (or $\eta(4)$) via creative telescoping?

Question Is it already known whether the $\zeta(4):=\sum_{n=1}^{\infty}1/n^4$ accelerated convergence series $(1)$, proved for instance in [1, Corollaire 5.3], could be obtained by a similar ...
16
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0answers
740 views

Convergence/Divergence of infinite series $\sum\limits_{n=1}^{\infty} \frac{1}{n^{1+\left|{\cos n}\right|}}$

It is well known that $ \displaystyle\sum_{n=1}^{\infty} \frac{1}{n}$ is divergent while $ \displaystyle\sum_{n=1}^{\infty} \frac{1}{n^{1+\epsilon}}$ is convergent for a fixed positive value of $\...
15
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0answers
245 views

The sequence $\,a_n=\lfloor \mathrm{e}^n\rfloor$ contains infinitely many odd and infinitely many even terms

PROBLEM. Show that the sequence $\,a_n=\lfloor \mathrm{e}^n\rfloor$ contains infinitely many odd and infinitely many even terms. It suffices to show that the terms of the sequence $$\,b_n=\mathrm{e}^...
14
votes
0answers
547 views

Asymptotic related to the infinite product of sine

The amount is somewhat complicated ($x$ is a constant): $$S_n=\sum_{k=1}^n\ln\left(1-\frac{\sin^2\big(x/(2n+1)\big)}{\sin^2\big(k\pi/(2n+1)\big)}\right)\tag{*}$$ I want to enrich my handy powerful ...
14
votes
0answers
633 views

On the weak and strong convergence of an iterative sequence

I have some difficulties in the following problem. I would like to thank for all kind help and construction. Let $H$ be an infinite dimensional real Hilbert space and $F: H\rightarrow H$ be a ...
13
votes
0answers
132 views

How much can we rearrange a series?

There's a well-known result that if $\sum a_n$ is conditionally convergent, then for any real $c$ there exists a permutation $\pi:\mathbb{N} \to \mathbb{N}$ such that $\sum a_{\pi(n)} = c$. A ...
13
votes
0answers
241 views

Number as the sum of digits of some degree

We will say that the measure of a number is equal to the maximum degree in which it is possible to represent a number in the form of a sum of digits copied (You can not rearrange the numbers). For ...
12
votes
0answers
128 views

Have “groupy” numbers been studied before?

In number theory, a positive integer $n$ is called highly composite if it has more divisors than any smaller positive integer. This notion has been studied by several notable mathematicians; for ...
11
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0answers
150 views

Is there an analytic solution for such problem?

Given function $$f_n(x) = \cos x - (\cos \cos x) + (\cos \cos \cos x) - (\cos \cos \cos \cos x) + \dots + (-1)^{n-1} \underbrace{ \cos \cos \dots \cos }_n x,$$ where $n \in \mathbb{N}$ and $\...
11
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0answers
238 views

The famous root of $x(1 - x^{12})^2(1 - x^{24})^2 = (1 - x^6)^7(1 - x^8)^4$

I. Let $n=2$ and $x = \frac1\phi$ with golden ratio $\phi$. Then it satisfies, $$x(1 - x^{12})^2(1 - x^{24})^2 = (1 - x^6)^7(1 - x^8)^4\tag1$$ $$x(1 - x^{12})^2 = (1 - x^4)^\color{blue}2(1 - x^6)^3\...
11
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0answers
202 views

Which Fourier series are “legal”?

Let's consider a continuous function $f(x)$ and real numbers $\lambda_n=(\alpha+\beta n)\pi$ where both $\alpha$ and $\beta$ are integers. In any interval $I$, is it true that $$ f(x)=\sum_{n\geq 0}...
11
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0answers
347 views

Combinatorial Proofs of Real Analysis Identity

In this question, a proof using real analysis is given of the following identity: $$ \sum_{n=1}^{\infty} \frac{(n-1)!}{n \prod_{i=1}^{n} (a+i)} = \sum_{k=1}^{\infty} \frac{1}{(a+k)^2}$$ Is there a ...
11
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0answers
208 views

How to prove that my sum coincides with a sequence on the Online Encyclopedia of Integer Sequences?

I have a topological space $X$ whose reduced $\bmod 2$ Betti numbers (that is to say, the dimension of the $\bmod 2$ reduced homology) I computed to be $$\small \dim \tilde{H}_t(X; {\mathbb{Z}}_2) = \...
10
votes
0answers
316 views

How many primes does this sequence find?

The sequence in question is: $$S=\left\{\int_0^1\pi(x)\pi(1-x)dx,\int_0^2\pi(x)\pi(2-x)dx,...\right\},$$ where $\pi(x)$ is the prime counting function. I don't know how to check this for an ...
10
votes
0answers
185 views

A conjecture about the connection between a Penrose tiling and the Fibonacci word fractal

Consider the Penrose tiling $P3$, inflated up to $6$ generations: We draw a line passing through the center of the tiling (red dot) and the outer vertex of the rightmost starting tile (black dot). ...
10
votes
0answers
251 views

Odd values for Dirichlet beta function

Hello there I want to find a proof for the generating formula for odd values of Dirichlet beta function given by wikipedia: link I searched MSE and didnt find something similar. My try was to start ...
10
votes
0answers
339 views

Does the sequence $x_0=12$ , $x_{n+1}=x_n^2+1$ contain a prime?

I wonder whether the sequence defined by $$x_0=12$$ $$x_{n+1}=x_n^2+1$$ for all non-negative integers $n$ contains a prime number. The following table shows from left to right : The index $n$ , the ...
10
votes
0answers
182 views

Can the sum of powers of the first primes be a square?

Let $p$ be a prime and $u\ge 1$ be a positive integer. Define $$\begin{align} S(p,u) &:= \sum_{q\text{ prime, }q \le p} q^u \\ &= 2^u+3^u+\cdots +p^u\end{align}$$ I wonder whether $S(p,u)$ ...
10
votes
0answers
602 views

Calculating growth rate of a population of Minecraft chickens

I have a rather strange question (for this Stack Exchange anyway). It felt too mathematical to ask elsewhere. If this is out of place here, please let me know. A chicken in Minecraft lays eggs; ...
10
votes
0answers
194 views

Can (linear) differential equations of infinite order be recast into equations of first order?

In most analysis courses one sees that differential equations of order $n$ are basically a subset of higher dimensional differential equations of order $1$, for example the equation: $$f^{(n)}(t)=F\...
10
votes
0answers
433 views

Extension of the Jacobi triple product identity

The Jacobi triple product identity is: $$\prod\limits_{n=1}^{ \infty }(1-q^{2n})(1+zq^{2n-1})(1+z^{-1}q^{2n-1})=\sum\limits_{n = - \infty }^ \infty z^n q^{n^2} $$ I would like to extend the idea for ...
9
votes
0answers
153 views

A curious coincidence in the series representation of $\zeta(7)$

Let $\zeta(n)$ denote the Riemann Zeta function defined for positive integers $n$ as usual by: $$ \zeta(n)=\sum_{m=1}^{\infty} \frac{1}{m^n}. $$ It is currently unknown whether there exists a series ...
9
votes
0answers
282 views

Evaluate $\sum_{n=0}^{\infty} \frac{n!}{(n^2)!}$

I'm interested in a method of evaluating $\sum_{n=0}^{\infty} \frac{n!}{(n^2)!}$. If there was a linear equation with leading coefficient $1$ in the denominator or a quadratic with leading ...
9
votes
0answers
135 views

Closed form for $\sum_{n=1}^\infty \frac{1}{P(n)}$, where $P(n)$ is the partition function.

Is there a closed form for the following infinite series? $$\sum_{n=1}^\infty \frac{1}{P(n)}=1+\frac{1}{2}+\frac{1}{3}+\frac{1}{5}+...$$ where $P(n)$ is the partition function.
9
votes
0answers
241 views

Invariant functions on the space of finite sequences of reals

Let $S$ be a space of all finite sequences of real numbers (we don't endow it with metric or topology in general). Before asking the main question, some notation. 1. For each $\mathbf s\in S$ we ...
8
votes
0answers
140 views

Triple sum $\sum\limits_{a=1}^{\infty} \sum\limits_{b=1}^{\infty} \sum\limits_{c=1}^{\infty} \frac{\cos a \cos b \cos c}{a^2 + b^2 + c^2}$

We have poor water heating system in our countryside house (currently it takes 4 hours to warm up the water), and my father has decided to improve it; he bought a water tank and placed it up in the ...
8
votes
0answers
89 views

How to prove using elementary methods that this function is everywhere continuous but nowhere differentiable?

Let $f$ be the function defined on all of $\mathbb{R}$ by the formula $$ f(x) \colon= \sum_{n=0}^\infty \frac{1}{2^n} \cos \left( 3^n x \right). $$ How to show (rigorously but through elementary ...
8
votes
0answers
88 views

A relation concerning the “sum of squares” counting function $r_2(n)$

Let $r_2(n)$ denote the number of ways in which a positive integer $n$ can be expressed as the sum of squares of two integers. Here the sign as well as order of summands matters. Also by convention we ...
8
votes
0answers
266 views

Interesting sequence involving prime numbers jumping on the number line.

Udpate 4: Trying to characterize finite and infinite cycles. Update 3: All primes $a_0\ge29$ seem to either have infinite cycles or finite non-terminating cycles that converge to infinite cycles of ...
8
votes
0answers
138 views

Does this “almost all integers in order” sequence have a closed form?

Can you help me define a formula for the following sequence (first $130$ terms) : ...
8
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0answers
250 views

The Chudnovsky pi formula $1/\pi$ revisited

Define the constants, $$A=163\cdot1114806\\B=13591409\\C=640320$$ Given the binomial coefficient $\binom{n}{k}$, then we have the pi formulas, $$\frac{1}{\pi} =\frac{12}{(C)^{3/2}}\sum^\infty_{k=0} \...
8
votes
0answers
242 views

Modified AGM: $a_{n+1}=\frac{a_n+b_n}{2}, \quad b_{n+1}=a_n+b_n-\sqrt{a_n b_n}$

The idea is as follows: generate a sequence from two numbers by subtracting their means from their sum, for example arithmetic and geometric means: $$a_{n+1}=a_n+b_n-\frac{a_n+b_n}{2}=\frac{a_n+...
8
votes
0answers
330 views

A monotonically increasing series for $\pi^6-961$ to prove $\pi^3>31$

This question is motivated by Why is $\pi$ so close to $3$?, Why is $\pi^2$ so close to $10$? and Proving $\pi^3 \gt 31$. I. $\pi$ and $\pi^2$ There are series with all terms positive for $\pi-...
8
votes
0answers
268 views

How to sum up this series and simplify yet another one?

Primarily, I would like to know what could be done with this series: $$ \sum_{n=2}^{\infty}\frac{n^3}{(n^2-1)^3}\left(\frac{n-1}{n+1}\right)^{2n}$$ As hardmath says in his comment, the series ...
8
votes
0answers
284 views

Name of a certain set

I want to know if there is any already-standard way to refer to the sets described as follows. For a set $X$, let $-X = \{-x: x \in X \}$; call it the negative of $X$. Take the set of all primes in $\...
8
votes
0answers
378 views

A sequence similar to the Catalan numbers

The $n$-th Catalan number $c_n$ has the closed form $\frac1{n+1}\binom{2n}{n}$ and follows the recursion $c_n = \sum\limits_{i = 0}^{n-1} c_{n-1-i}c_i$ I am interested in the quantity $e_n$ which ...
7
votes
0answers
134 views

The sine cardinal function and $F_1 = F_2 = F_3 = F_4 = F_5 = F_6 = 0$

Define the function, $$F_n=\frac12-\int_0^\infty \frac{\sin^n x}{x^n}\,dx+\sum_{x=1}^\infty \frac{\sin^n x}{x^n}\tag1$$ where $\rm{sinc}^n(x)=\frac{\sin^n x}{x^n}$ is the sine cardinal function. We ...
7
votes
0answers
85 views

Analogous “Dark Sector” Trigonometric (and Hyperbolic) Functions

Existing Definitions: $$\zeta(n)=\sum_{k=1}^\infty \frac{ 1 }{k^n}$$ $$\lambda(n)=\sum_{k=1}^\infty \frac{ 1 }{(2k-1)^n}=\frac{\left(2^n-1\right)}{2^n}\zeta (n)$$ $$\eta(n)=\sum_{k=1}^\infty \frac{(-1)...
7
votes
0answers
150 views

Recurrence $a_{n}=a_{\lfloor 2n/3\rfloor}+a_{\lfloor n/3\rfloor}$

I am considering the sequence $$a_n=a_{\lfloor 2n/3\rfloor}+a_{\lfloor n/3\rfloor}$$ with $a_0=1$, and I would like to calculate the limit $$\lim_{n\to\infty} \frac{a_n}{n}$$ I have seen this famous ...
7
votes
0answers
120 views

Gap in spiral sequence

OEIS sequence A272573 describes a sequence generated in the following way: Start a spiral of numbers on a hexagonal tiling, with the initial hexagon as a(1) = 1. a(n) is the smallest positive ...
7
votes
0answers
85 views

Determining finitude or infinitude from a simple geometric construction

Playing with a pencil on a checkered sheet I encountered this construction: 1) take a point $A$ on the grid and a point $B$ that is distant from $A$ $n=2,3,4...$ horizontal steps and $1$ vertical ...
7
votes
0answers
211 views

Check simple proof that $\lim\limits_{s\to0^+}\sum\limits_{n=1}^\infty(-1)^nf(n)^{-s}=-\frac12$ if $f>0$, $f''\le0$ and $f(+\infty)=+\infty$

$f:[1,+\infty)\to \mathbb{R}_+$ satisfies $\ f''\leq0,\ f(+\infty)=+\infty $. Prove $$\lim_{s\to0^+}\sum_{n=1}^\infty (-1)^n[f(n)]^{-s}=-\frac{1}{2}$$ Some of my friends showed me this question ...
7
votes
0answers
196 views

Generating function of the sequence $\binom{2n}{n}^3H_n$

Generating functions of the sequences $\binom{2n}{n}^2H_n$ and $\binom{2n}{n}^2H_{2n}$, where $H_n$ is $n$-th harmonic number, are known in terms of elliptic integrals $$ \sum_{n=1}^\infty\binom{2n}{n}...