Questions tagged [sequences-and-series]
for questions about sequences and series, e.g. convergence, closed form expressions, etc. Note that there is a different tag for spectral sequences, and also note that MathOverflow is not for homework. Please consider consulting the online encyclopedia for integer sequences, if you are trying to identify a given sequence that you have found in your research.
1,731
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Interchange summation order in the limit of number of elements going to $\infty$
Considering the sum $\sum_{i=0}^{\infty}\sum_{j=0}^{\infty} a_{ij}$, in general we are not allowed to interchange the summation order (i.e. pass to $\sum_{j=0}^{\infty}\sum_{i=0}^{\infty} a_{ij}$) but ...
-1
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14
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Priming for the primes [closed]
I have to confess that most often my eyes begin to glaze over when someone starts discussing the prime numbers. However, my ears have perked up at times over the primes--maybe first when I learned of ...
2
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0
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333
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Combinatorics of iterated composition of noncrossing partition polynomials
A combinatorial problem arises in relating connected and disconnected Green functions associated to a "zero-dimensional" quantum field theory presented by Brezin, Itzykson, Parisi, and Zuber ...
2
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1
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153
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Bounds for the sequence $a(n)=a(n-1)+a(\lfloor n-n^A \rfloor)$
Related to the question about a(n)=a(n-1)+a(floor(n/2))
Let $A$ be real constant $ 0 < A < 1$.
Define the sequence $a(n)$ by $a(1)=1, a(n)=a(n-1)+a(\lfloor n-n^A \rfloor)$
(if you prefer take $a'...
2
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0
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206
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Question on globally convergent formulas for the Riemann zeta function $\zeta(s)$
Consider the following two formulas for $\zeta(s)$
$$\zeta(s)=\underset{K\to\infty}{\text{lim}}\left(\frac{1}{1-2^{1-s}}\sum\limits_{n=0}^K \frac{1}{2^{n+1}}\sum\limits_{k=0}^n \binom{n}{k} \frac{(-1)^...
6
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1
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Are these continued fractions for the "tails" of $\zeta(3)$ and of the Catalan constant known?
For polynomials $a=a(x)$ and $b=b(x)$, define the continued fraction $$f(a,b):=a(1)+ \lower 2pt\overset{\infty }{\underset{n=1}{\mathbb{\LARGE K}}}~\dfrac{b(n)}{a(n+1)}=a(1)+\cfrac{b(1)}{a(2) + \cfrac{...
1
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1
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86
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On the behaviour for the quotient involving Fermat numbers of $\frac{\psi(F_m)}{F_m}$ where $\psi(x)$ denotes the Dedekind psi function
In this post we denote the Dedekind psi function as $\psi(m)$ for integers $m\geq 1$. This is an important arithmetic fuction in several subjects of mathematics. As reference I add the Wikipedia ...
1
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1
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210
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Solving (or approximating) a certain delay differential equation
I'm interested in finding the (unique?) solution to the set of delay differential equations
$$f_w(w,x) = xf(w,w^2x)+w^3x^2f(w,w^4x), $$
$$f_x(w,x) = wf(w,w^2x)$$
With the initial condition $f(1,x) = e^...
0
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0
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68
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Dense subspace of square integrable functions on the complex disc
Denote by $L^{2}(D,(1-|z|^{2})^{a-1}|z|^{2b-2}dx dy)$ the set of square integrable functions on the complex disc $D= \lbrace z \in C, \; |z| <1 \rbrace$ with respect to the measure $(1-|z|^{2})^{a-...
1
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0
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Has the Collatz been investigated as a recursive function?
Does anyone ever write the Collatz conjecture as a single algebraic, recursive sequence? For example, a crude version might be:
$$
g(n+1)=\delta _{1,g(n)}+(1-\delta _{1,g(n)})*\left(\left(\frac{cos(\...
2
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0
answers
136
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Closed form for the limit $\lim\limits_{n\to\infty}\frac{1}{n}\sum\limits_{i=1}^n[\gcd(i,\lfloor\frac ni\rfloor)=1]$
If we define $f(n)$ as the number of coprime pairs $(i,\lfloor\frac ni\rfloor)$ for $i$ an integer from $1$ to $n$, then $f(n)\sim cn$ for a constant $c\approx 0.7883$.
Because $f(n)=\sum\limits_{i=1}^...
20
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2
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709
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Multiple roots of polynomials with coefficients $\pm 1$
Question P. Can a polynomial $P(x)=\sum_{n=0}^ma_nx^n$ with coefficients $a_n\in\{-1,1\}$ (and $P(1)=0$) have a multiple root in the interval $(\tfrac12,1)$?
Also I am interested in a similar question ...
9
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2
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387
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Rearrangement, conditional convergence, and "placid" permutations
This question came out of a conversation with my students about Riemann's rearrangement theorem, and the general problem of which permutations are "safe" w/r/t summing infinite series. It ...
17
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1
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Catalan's constant fast convergent series
NOTE. UPDATE 2 introduces proven series for Catalan's constant that is possibly the fastest currently known.
Working with some conjectured continued fractions that were published here, I have found ...
5
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1
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339
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Is this function rational?
Let
$$
F=\sum_{i\ge0}\frac1{(T+2)^i}\left(\frac T{T+1}\right)^{3^i}\in\mathbb F_3\left(\!\!\left(\frac1T\right)\!\!\right).$$
Does $F$ belong to $\mathbb F_3(T)$?
Here, truncations of the series do ...
1
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0
answers
190
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Closed form for partial sums of A103318
Let $a(n)$ be A103318, number of solutions $i$ in range $[0,n-1]$ to $i \equiv 0 \pmod {2^{n-i}}$: the sequence begins with
$$1, 1, 2, 1, 2, 2, 2, 1, 2, 2, 3, 1, 2, 2, 2, 1, 2, 2, 3, 2$$
Also let's ...
5
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1
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380
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Growth of sequence generated by recurrence relation
Consider the following recurrence relation:
$$T(n) = 0 \text{ when } n \leq 0\\
T(n) = 1+\sum_{p=0}^{\infty}T\left(\left\lfloor\frac{n}{2^p}\right\rfloor - 1\right)$$
The first few integers generated ...
0
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1
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$a(16n+k)=b(16n+k)-c(16n)$ for $n\geqslant0$, $0 < k < 16$ where $c(n)=b(n)-a(n)$
Let $a(n)$ be A339970 = A329697$($A019565$(2n))$: the sequence begins with
$$0, 1, 1, 2, 2, 3, 3, 4, 2, 3, 3, 4, 4, 5, 5, 6, 2, 3, 3, 4, 4$$
Also let's consider
$$\ell(n)=\left\lfloor\log_{2}(n)\right\...
7
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1
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A differential equation governing compositional inversion
Looking for references for the following theorem.
Given the formal Taylor series/exponential generating function
$$T(z) = \sum_{n \ge 1} a_n \; \frac{z^n}{n!},$$
for which the indeterminates $a_n$ and ...
0
votes
1
answer
215
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Series of reciprocals of smooth numbers
Be a non-empty set of primes $A $. Let us define $A^{\otimes}$ as the set of numbers smooth over $A$, that are the naturals having all their prime divisors in $A$ (where $1$ is arbitrarily considered ...
5
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1
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177
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Reference request for a certain exponential series
I recently encountered the series $$\sum_{d \in \mathbb{Z}} e^{-t^d}t^{kd},$$ for real $0<t<1$ and $k$ a positive integer. It converges because roughly speaking, when $d$ is large and positive ...
2
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0
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61
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Odious twin locations related to the sequence based on $d(n) = n-d(d(n-1))-d(d(n-2))$
Let $\operatorname{wt}(n)$ be A000120, i.e. the number of $1$'s in binary expansion of $n$ (or the binary weight of $n$).
Let $a(n)$ be the sequence of numbers $k$ such that $\operatorname{wt}(k)\...
1
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1
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107
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Periodic sequences of integers generated by $a_{n+1}=\frac{\operatorname{rad}(pa_{n})}{p}+\frac{\operatorname{rad}(qa_{n-1})}{q}$
Let's define the radical of the positive integer $n$ as
$$\operatorname{rad}(n)=\prod_{\substack{p\mid n\\ p\text{ prime}}}p$$
and consider the sequence
$$a_{n+1}=\frac{\operatorname{rad}(p\cdot a_{n})...
4
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0
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Binary iterations, Fibonacci numbers and permutation of natural numbers
Let $\operatorname{wt}(n)$ be A000120, i.e. the number of $1$'s in binary expansion of $n$ (or the binary weight of $n$).
Also let's consider
$$\ell(n)=\left\lfloor\log_{2} n\right\rfloor$$
and
$$T(n,...
0
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0
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111
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Parseval identity extension?
I have stumbled upon the following three-dimensional series:
$$\Lambda_p = \sum_{\underline{n}} \left(\frac{\left|n_1\right|}{\left|\left|\underline{n}\right|\right|_2}\right)^p \left|\hat{f}(\...
1
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1
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Is this long closed form for pi trivial?
With the help of wolfram alpha we got very long closed form
for $\pi$ in terms of algebraic numbers, logarithms of algebraic
numbers and $cot^{-1},coth^{-1}$ which can be expressed as logarithms.
From ...
9
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3
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Why mpmath computes $\sum_{n=2}^\infty (-1)^n\log(n)=\log\left(\frac1 2 \sqrt{2} \sqrt{\pi}\right)$
Working with precision 500 decimal digits, mpmath in sage computes:
$$\sum_{n=2}^\infty (-1)^n\log(n)=\log\left(\frac1 2 \sqrt{2} \sqrt{\pi}\right).\tag{1}\label{1}$$
We believe the LHS of \eqref{1} ...
4
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3
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Surprisingly long closed form for simple series
For natural $A$ define
$$ f(A)=\sum_{n=1}^\infty \frac{1}{A^n}\left(\frac{1}{An+1}- \frac{1}{An+A-1}\right)$$
$f(A)$ is BBP (Bailey-Borwein-Plouffe) formula and allows digit extraction in base $A$.
...
5
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1
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Infinite dimensional involutions: infinitely large sets of multivariate polynomials self-inverse under self-substitution
Examples of infinite dimensional involutions
Edit 2/25/23, as suggested by YCOR below: (Start)
The first return on a Google search on involution--from late Latin 'a rolling up'--gives the Oxford ...
2
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0
answers
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The Laplace transform and the Lagrange compositional inversion formula
I'm looking for references which derive the Lagrange inversion formula, given below (in bold), for the Taylor series coefficients of the compositional inverse of a function $f$ analytic at the origin ...
2
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1
answer
299
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What's the fastest way to compute $\log n$ for $n>1$?
As it is well known, if $|x|<1$ then we can compute $\log(1+x)$ by the Taylor series
$$\log(1+x)=x-\frac{x^2}2+\frac{x^3}3-\cdots.$$
Thus, to compute $\log n$ with $n>1$, we may employ the ...
1
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1
answer
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Sequence of reals such that $x_{n+1}\leq ab^{n}x_{n}^{1+s}$ converges to $0$?
Let $\{x_{n}\}_{n=0}^{\infty}$ be decreasing sequence of non-negative reals. Suppose that there exist constants $a, s>0$ and $b>1$ such that $$x_{n+1}\leq ab^{n}x_{n}^{1+s}$$ and $$x_{0}\leq a^{-...
6
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0
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Is there a residue sum formula in quaternionic analysis?
In complex analysis, there is a formula involving the residues of complex functions that one can employ to find the value of certain infinite series.
If the function $f: \mathbb{C} \to \mathbb{C} $ ...
1
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1
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A conjectural identity involving infinite series
Recently I formulated the following curious conjecture based on my computation.
Conjecture. For all $|x|>1$, we have the identity
$$\sum_{k=0}^\infty\frac{\sum_{j=0}^{k}\binom{2k+1}{2j}(1-x)^jx^{k-...
2
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1
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One series converges iff the other converges
In Show that $\sum\limits_pa_p$ converges iff $\sum\limits_{n}\frac{a_n}{\log n}$ converges it is said that this sequence of partial sums converges
$$
\begin{split}
\sum_{1<n\leq N}\frac{a_{n}}{\...
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0
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$\lim_{x\to \infty} \left(\sum_{n\leq x} (\log n)^k/n - \int_1^x (\log t)^k/t\right) = \text{?}$
It is easy to see (by Euler-Maclaurin, say, or just by thinking of a graph) that
$$\lim_{x\to \infty} \sum_{n\leq x} \frac{(\log n)^k}{n} = \int_1^x \frac{(\log t)^k}{t} + C + O\left(\frac{(\log x)^k}{...
4
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0
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Karamata's Abelian/Tauberian Theorem in the complex plane
The following result is well known (a particular case of Karamata's Tauberian Theorem for Power Series in Corollary 1.7.3 of Regular variation by Bingham, Goldie & Teugels):
Fix $c, \rho>0$. If ...
1
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0
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Self-referencing recurrence relation with exponential
I have the self-referencing recurrence relation
$$
d(0) = 0
$$
$$
d(1) = a
$$
$$
d(n+1) = d(n) + a*e^{-(\frac{d(n)-n*c}{b})^4}
$$
Written as a sum:
$$
d(N) = a*\sum_{n=0}^{N}e^{-(\frac{d(n)-n*c}{b})^...
2
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1
answer
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Are Chebyshev polynomials a Schauder basis of $\mathrm{Lip}[-1,1]$?
It is known that every Lipschitz function $f \colon [-1,1] \to \mathbb R$ can be expressed as a series in the Chebyshev polynomials $$f = \sum_{n = 0}^\infty a_n T_n $$ which is absolutely convergent ...
6
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0
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94
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q-binomial-like series with exponentials defining probability distribution
Recently I encountered the series
$$f(d) = \frac{1}{(t;t)_\infty} \sum_{k=0}^\infty \frac{(-1)^k t^{\binom{k}{2}}}{(t;t)_k} e^{-t^{d-k}}$$
where $(t;t)_n = \prod_{i=1}^n (1-t^i)$, and $0 < t < 1$...
1
vote
1
answer
112
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Numbers $m$ for which coefficients of the polynomial $p(m,x)$ are relatively prime
From A248667:
The polynomial $p(n,x)$ is defined as the numerator when the sum
$$1 + \frac{1}{nx + 1} + \frac{1}{(nx + 1)(nx + 2)} + \cdots + \frac{1}{(nx + 1)(nx + 2)\cdots(nx + n - 1)}$$
is written ...
0
votes
1
answer
171
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Number of positive integers $k$ such that there exists a nonnegative integer $m$ with $k + k^m = n$
Let $a(n)$ be the number of positive integers $k$ such that there exists a nonnegative integer $m$ with $k + k^m = n$.
The sequence begins
$$0, 1, 1, 2, 1, 3, 1, 2, 1, 3, 1, 3, 1, 2, 1, 2, 1, 3, 1, 3$$...
0
votes
0
answers
352
views
Is $\lim_{x\to\infty}\sum_{n=-\infty}^{\infty}\frac{x}{n^2+x^2}=\pi$?
It seems that
$$\lim_{x\to\infty}\sum_{n=-\infty}^{\infty}\frac{x}{n^2+x^2}=\pi.$$
But I can't prove it. I cannot prove that the function is decreasing in $x$ either.
1
vote
0
answers
71
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Geometric series involving the Laguerre polynomials
Let put $\alpha=5$ and $x=3$. Consider the following set given by
$$M=\lbrace \; n \in N, \; \; 0 < |L_{n}^{5}(3)| < 1 \; \rbrace$$
Where $L_{n}^{\alpha}(x)$ is the generalized Laguerre ...
3
votes
1
answer
91
views
Tauberian lower bound for a series
Let $(a_n)_{n\in\mathbb{N}}$ be a sequence of positive number such that $\sum_n a_n < +\infty$ (i.e. $a_n \in \ell^1$) but $\sum_n r^n a_n = +\infty$ for every $r > 1$.
Given $\sigma \in (0,1)$, ...
3
votes
1
answer
267
views
Linear combinations of geometric series
Consider the uncountable-dimensional vector space $V$ consisting of finite linear combinations of infinite sequences of the form $(1,z,z^2,z^3,\dots)$ with $z \neq 1$ in $\mathbb{C}$. Since the ...
9
votes
2
answers
339
views
Asymptotics of a quadratic recursion
Consider the sequence defined by
\begin{align}
c_0 &{}= 1 \\
c_n &{}= 2\,n\,c_{n-1}-\frac{1}{2}\sum_{m=1}^{n-1}c_m\,c_{n-m}.
\end{align}
How can you prove that it has the following asymptotics ...
1
vote
1
answer
153
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Upper bound on double series
We consider the sum
$$ \sum_{m \in \mathbb Z^2} \frac{1}{(3 m_1^2+3m_2^2+3(m_1+m_1m_2+m_2)+1)^2}. $$
Numerically, it is not particularly hard to see that the value of this series is well below $4$, ...
2
votes
1
answer
160
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As-closed-as-possible formula for an integral and/or sum
I need to find the solution of this integral:
$$\int^\pi_{-\pi}{d\varphi\,{}\frac{\Gamma(n,2\pi\varphi)}{(1+ia\varphi)^n}},\tag{1}\label{1}$$
where $a\in(0,1)$ and $n$ is a positive integer (not zero)....
8
votes
3
answers
1k
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Series solution for general trinomial
Consider the equation
$x^5-2x^2+z=0$
How do you derive the Lagrange inversion theorem series solution for it? I know it exists because the answer is here for any trinomial https://arxiv.org/pdf/0910....