# Questions tagged [power-series]

The power-series tag has no usage guidance.

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### Finding Laurent series of function (z^2 + 1)/z [on hold]

Can someone help me finding the laurent series of (z^2 +1)/z?

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**2**answers

135 views

### Density of Lacunary Functions

I'm curious whether lacunary functions (functions in the complex plane that are holomorphic in some open ball about the origin, but cannot be analytically extended past that ball) are typical or the ...

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**1**answer

72 views

### Laplace transform of the tetration (integral or series)

How to get some insight in the following integral:
\begin{equation}
\mathcal{I}(s)=\int_0^\infty x^{-x}e^{sx}\text{d} x
\end{equation}
where $s$ is real (and the lower integration bound may be set ...

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59 views

### Bivariate power series as rational function

Suppose we have a bivariate power series of the form
$$\sum_{i}\sum_j a_{i,j} t^i s^j,$$
where for every fixed value of $i$ the corresponding univariate power series in $s$ is a rational function. Are ...

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56 views

### On the necessitation of $(-1)^n$ within the series expansion of $f(x)$ concerning the usage of Ramanujan's Master Theorem

Ramanujan's well known Master Theorem states that the series expansion of the transformed function $f(x)$ has to be in form of
$$f(x)~=~\sum_{n=0}^{\infty}(-1)^n\frac{\phi(n)}{n!}x^n\tag1$$
...

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96 views

### Calculating the Taylor series, given a functional equation

I have two functions, whose Taylor series about infinity are given by
$$ f(z) = \frac{1}{z} + \sum_{n=2}^{\infty} \frac{A_k}{z^k}, \quad g(z)=\frac{1}{z} + \sum_{n=2}^{\infty} \frac{B_k}{z^k} $$
and ...

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**2**answers

300 views

### Partitions, $q$-polynomials and generating functions

Recall the integer partition function $P(n)$ with generating function
$$\sum_{n\geq0}P(n)x^n=\prod_{k=1}^{\infty}\frac1{1-x^k}.$$
Let $[n]_q=\frac{1-q^n}{1-q}$ denote the $q$-analogue of the integer $...

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**2**answers

605 views

### Searching for a proof for a series identity

The below identity I have found experimentally.
Question. Is this true? If so, may you provide a "slick" (or any) proof.
$$6\sum_{k=1}^{\infty}\frac{k^2q^k}{(1-q^k)^2}+12\left(\sum_{k=1}^{\infty}...

**7**

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**1**answer

229 views

### Descartes' rule of signs for infinite series

Consider the function given by
$$f(x)=1-a_1x-a_2x^2-a_3x^3-\cdots$$
where each $a_k\geq0$ and some $a_j>0$. If $f(x)$ is a polynomial then Descartes' Rule of signs tells us there is exactly one ...

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### Cohen structure theorem with explicit equations

By Cohen structure theorem, a complete regular equicharacteristic Noetherian local ring is isomorphic to a power series. In particular, this should hold for finite extensions of power series $k[[t]][\...

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311 views

### Are there integral solutions for $(2a-1)(2^{(b+c)}-3^c )=2^b-1$?

Can anyone prove this assertion? Or at least suggest a method of attack? It has come up in my research.
There do not exist $a,b$ and $c$ such that$$
(2a-1)(2^{(b+c)}-3^c )=2^b-1
$$where $a>0,b&...

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**1**answer

423 views

### Is $K[[x_1,x_2,\dots]]$ an $\mathfrak m$-adically complete ring?

I asked this question on Mathematics Stackexchange (link), but got no answer.
Let $K$ be a field, let $x_1,x_2,\dots$ be indeterminates, and form the $K$-algebra $A:=K[[x_1,x_2,\dots]]$.
Recall ...

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147 views

### power series and roots of unity

Let $p$ be an odd prime and $X$ and $Y$ be subsets of $p^{th}$ roots of unity, $|X|=|Y|=n,X\neq Y.$ Let $f(t)=\sum_{x\in X}x^{t}-\sum_{y\in Y}y^{t}$. If $f(t)=at^k+o(t^k)$ is the power series ...

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61 views

### Rationality of power series whose coefficients are the ranks of a sequence of matrices

Recently, I stumbled several times about the problem to decide whether a certain formal power series
$$ f = \sum_{n=0}^\infty d_n T^n \in \mathbb{Q}[\![T]\!]$$
is actually a rational function, where ...

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70 views

### Relation between coefficients of expansions

Related to Relations between coefficients of expansions of a rational function at 0 and infinity
I commented at the linked question that the question seemed less about what happened "at infinity", ...

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**1**answer

447 views

### Relations between coefficients of expansions of a rational function at 0 and infinity

This question goes in the bucket of "this must be well known, but I don't see it and am not sure where to look it up."
Given two Laurent power series $A(t)=\sum_{k>N}a_kt^k$ and $B(t)=\sum_{k>M}...

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**2**answers

238 views

### Algebraic power series over $\mathbb{F}_2$ as roots of polynomials of special form

Let $F = \mathbb{F}_2$ be the field with two elements. I will denote the rings of polynomials and formal
power series over $F$ as $F[t]$ and $F[[t]]$ respectively. Suppose that $x \in F[[t]]$ is ...

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**1**answer

61 views

### Notions of convergence over extensions of finite fields

Let $\displaystyle Q_p[x] = \left\{\frac{p(x)}{q(x)} \mid \, p(x),q(x) \in \mathbb{F}_p[x], \, q(x) \neq 0 \right\}$ denote the field of fractions extending $\mathbb{F}_p[x]$. If we consider the ...

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55 views

### Upper bound on the modulus of a power series and concentration inequalities for empirical processes

This is a research question I encountered when I as studying solutions of
Lebesgue-Stieltjes integral equations. It is related to a new statistical
method I am developing (which I cannot expose now) ...

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**1**answer

345 views

### References on Power Sums

Consider a recent arXiv preprint 1805.11445. The author of 1805.11445 has done an overview of classical problem of simplifying of power sum
$$\sum_{1\leq k\leq n}k^m, \ (n,m)\geq 0, \ m=\mathrm{const}...

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147 views

### On an isomorphism between $p$-adic power series and an inverse limit

Let $K$ be an extension field of $\mathbb{Q}_p$, let $O$ be the ring of integers of $K$, and let $P$ be the maximal ideal of $O$.
If $K$ is a finite extension of $\mathbb{Q}_p$, there is the well-...

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195 views

### Integral element over p-adic power series

Let $p$ be a prime number. and $R[[X]]$ be the ring of formal series with coefficients in a $p$-adic field $R$. Let $\Lambda=\mathbb{Z}_p[[X]]$.
Question 1) Does there exist an explicit description ...

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**2**answers

65 views

### Confluent Heun Equation

Does anyone know any source in which I could find a recurrence relation for the coefficients of the series solution of the Confluent Heun Equation
$$y''+\left( {\gamma\over z}+{\delta\over z-1}+\...

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**1**answer

143 views

### Cancellation problem for Laurent polynomial rings and power series rings

Throughout, let $k$ be an algebraically closed field. For two $k$-algebras $A,B$ let us write $A \cong_k B$ to mean that $A,B$ are isomorphic as $k$-algebras.
It is known that if $A$ is an integral ...

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148 views

### Should the power series solution to $y' = y, y(0) = 1$ be obvious? [closed]

My Understanding:
I would derive the Poisson random variable as follows:
I consider an experiment which consists of a continuum of trials on an interval $[0,t)$. The result of the experiment takes ...

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88 views

### Deciding when certain elements of $L[[x]]$, coming from recursions, are algebraic over $L(x)$

Let $L$ be a finite field of characteristic $2$. Suppose that for some $k > 0$ we are given elements $A(0),\, A(1), \dots, \, A(k-1)$ and $c(0),\, c(1), \dots,\, c(k-1)$ of $L[t]$. Define $A(n)$ ...

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### Are $\mathbb C$ , $\mathbb C[X]$ definable in $\mathbb C[[X]]$?

Let $L$ be a first-order language and $M$ be an $L$-structure. Let $D \subseteq M^n$ . Let us say $D$ is definable in $M$ if for some finite set (possibly empty) $A=\{a_1,...,a_m\} \subseteq M$ and ...

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108 views

### Non-recursive expression for coefficients of the derivative of the logarithm of a power series

Let $f :(-1,1) \to \mathbb{R};\ \ f(x)=\sum_{n=0}^\infty a_n x^n$ be an analytical function expressible as a power series.
Also, let
$$g : (-1,1) \to \mathbb{R}; \ \ g(x)=\frac{d}{dx} \log{f(x)} = \...

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131 views

### Boundary behavior of power series vs. boundedness of partial sums

Let $f\left(z\right)=\sum_{n=0}^{\infty}a_{n}z^{n}$ be a power series with $0$s and $1$s as its coefficients ($a_{n}\in\left\{0,1\right\}$ for all $n$) with a radius of convergence of $1$. I call such ...

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### Characterizing positivity of formal group laws

The formal group law associated with a generating function $f(x) = x + \sum_{n=2}^\infty a_n \frac{x^n}{n!}$ is $$f(f^{-1}(x) + f^{-1}(y)).$$ In my thesis, I found a large number of examples of ...

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215 views

### On the coherence of $K[[X_1,X_2,…]]$

Recall that a commutative ring is coherent if every finitely generated ideal is finitely presented, or equivalently if every submodule of every finitely generated module is finitely presented.
Let $A ...

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291 views

### Solution to algebraic equations over $\mathbb{C}$ and $\mathbb{C}[x]$

$t^n=a$, we get one solution to the equation:
$$t=e^{\frac{1}{n}\int^a_1 \frac{1}{x}}$$ generalizing this result by replacing the exponential with an elliptic modular function and the integral with ...

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**1**answer

218 views

### Does $\int_{0}^{\infty}e^{-xz}\sum_{n=0}^{\infty}a_{n}\frac{x^n}{n!}dx$ converge for $z>0$ with $a_{n} > n! $, for $ n>1$? [closed]

Let $g$ be exponential generating function such that $g(x)= \sum_{n=0}^{+\infty}a_{n}\frac{x^n}{n!}$ extended by analytic continuation along $\mathbb{R+}$ and has a positive radius of convergence. We ...

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144 views

### Is this a Borel summable $ S = \sum_{k=0}^\infty (-1)^k (k!)a_k $ with $ a_k$ alternating sequence?

let $ S = \sum_{k=0}^\infty (-1)^k (k!)a_k $ a divergent series such that $b_k=(-1)^k (k!)a_k >0 $ for $k>1$ , and $b_k$ signed this from $k=1$ to $20$ ,The asymptotic of the titled series ...

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137 views

### What is a sufficient condition for summability of formel power series? [closed]

There are several kind of summability , i accrossed differents conditions for applying for example Borel summation or laplace transform which let me mixed and confused , really i don't know if i have ...

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133 views

### What can we know about “the half” of the generating series of Bessel function

I am interested in the series
$$\sum_{n\geq 1}I_n(x)\lambda^n$$
which is not the full generating series of the modified Bessel function of the first kind because it starts from $n=1$ and not at $-\...

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**1**answer

120 views

### Functions on a field representable by Hahn series?

It is well known (see here for example) that a function over $\mathbb{R}$ is representable by a power series iff its analytic continuation to $\mathbb{C}$ is holomorphic on some open subset of $\...

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87 views

### Differentiation and endpoints of power series [closed]

It is known that the power series
$\sum_{n=0}^\infty a_n x^n$
and $\sum_{n=0}^\infty n a_n x^{n-1}$
have the same radius of convergence $r$. Is it true that
if $r<\infty$ and $\sum_{n=0}^\infty ...

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164 views

### Formal multidimensional Taylor series expansion over commutative rings

If $F:V\to W$ is a smooth at $a\in V$ function between finite-dimensional vector spaces over $\mathbb{R}$, then we have
$$
F(x) = \sum_{k=0}^N\frac{1}{k!}(D^kF)(a)[(x-a)^{\otimes k}]+\text{remainder},
...

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273 views

### What's the summation of formal series $\sum_{n\geq0}\binom{n\delta}{n}x^n$？

$\delta$ is a positive number. Is this Taylor expansion of some function?

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163 views

### Asymptotic growth of the of Taylor coefficients of the inverse of a function

Let $f(x)=\sum_{n\geq 1} c_n\cdot x^n$ be a function given by a power series. Further there is some $\alpha >1$ such that for all $n$, $c_n = \Theta(1/n^{\alpha})$. What can one say about the ...

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**1**answer

77 views

### Bounds for the coefficients of the even entire function with positive coefficients

Suppose that the function $f$ is defined by
$f(z) = \sum_{j=0}^\infty a_{2j} z^{2j}$ where $a_{2j} \ge 0, z \in \mathbb{C}$. My questions are the following:
First I want to check this point: if we ...

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311 views

### Is there any closed form expression for $\sum_{k=2}^\infty(-1)^k \left(- \frac{1}{2}\right)^{\frac{k(k+1)}{2}}$?

Is there any closed form expression for the following serie?
$$\sum_{k=2}^\infty(-1)^k \left(- \frac{1}{2}\right)^{\frac{k(k+1)}{2}}$$
Or at least a proof that it is an irrational number. The ...

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68 views

### Getting the singularities of a function defined by a series

I am trying to locate the singularities that a linear transformation creates. I will not try to motivate this question, since it is already quite far from its starting point.
So, the question is the ...

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**1**answer

139 views

### Yet another question about unrestricted partitions

I posed a question called "A Product Related to Unrestricted Partitions". As it stands it is too hard. Here's another variation which is easier to search for and hopefully might shed some light on ...

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**1**answer

253 views

### An elementary question about a sequence of numbers

Let $\lambda_n$ be an increasing and unbounded sequence of positive real numbers and $a_n$ be a sequence of real numbers such that
$$\sum_{n=1}^\infty a_n \lambda_n^k=0 \ \ \text{ for all }\ \ k\geq ...

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**1**answer

239 views

### A strange (possible) fact about the Hecke operator T_3 in level 13 and characteristic 2

delta(z) + delta (13z) is a weight 12 modular form of level Gamma_0 (13). Let A in Z/2[[q]] be the mod 2 reduction of the Fourier expansion of this form. (The exponents appearing in A are the odd ...

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**1**answer

138 views

### An analogue of rational functions for Hahn series

For any field $k$, we have both the field $k(t)$ of rational functions (formal quotients of polynomials, i.e. the field of fractions of $k[t]$) and the field $k((t))$ of formal Laurent series (which ...

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147 views

### A constrained double summation

This is a question that I asked on Math StackExchange (see here), but I believe it is better to ask if any number theorist has encountered it before.
Consider two positive integer $(k,l)$ and they ...

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### Series representation of multiplication of two modified Bessel function

Series representation of multiplication of two Bessel function $J_{\mu}(az) J_{\nu}(bz)$ is in terms of sum of hypergeometric functions $_2F_1$, it given in book Treatise on Theory of Bessel Functions ...