Questions tagged [taylor-series]
Taylor series is a method to analyze functions as polynomials.
144 questions
-2
votes
0
answers
43
views
About the uniqueness of the Taylor polynomial
I'm in trouble understanding this theorem:
For a given function, differentiable n times at a given point $x_0$, there exist a unique polynomial $P_n$ (of degree $\le$n) such that
$$\forall \; k=0,...,...
- 1
3
votes
1
answer
138
views
Can the coefficients of a Taylor series be expressed as rational functions for an affine variety?
Let $k$ be an algebraically closed field and $V \subseteq k^n$ an affine variety corresponding to a prime ideal $P \subseteq k[t_1, \dots, t_n]$. For $x\in V$ let $O_x = \{p/q \mid p,q\in k[t_1, \dots,...
- 696
1
vote
0
answers
207
views
Jaw-breaking sum (related to quasi-analytic decompositions of unity — and Hörmander’s Lemma 1.3.6 from LinPDE vol.1)
$$F\left(\frac 1{\text e}\right) ≈ 1-\frac 1{\text e};\qquad\text{here }F(z)≔\sum_{k=1}^\infty \frac{(k-1)^{k-1}}{k!}z^k.$$
The match is at least with 800 decimal places (checked with ...
- 455
1
vote
1
answer
102
views
Does $C^{k,s-k}$ function with lipschitz lower order derivatives give a certain bound on the Taylor remainder?
Let $\Omega \subseteq \mathbb{R}$ be open (not necessarily an interval). Let $ s > 0$ and $k \in \mathbb{N}_0$ be such that $s \in (k, k+1]$. Suppose that
$f \colon \Omega \to \mathbb{R}$ is an ...
- 820
0
votes
0
answers
66
views
Uniformization and constructive analytic continuation of Taylor-Maclaurin series
Context. In their paper, "Uniformization and Constructive Analytic Continuation of Taylor Series", Costin and Dunne present a constructive method to greatly increase the accuracy of a ...
-4
votes
1
answer
197
views
How to express a quadratic polynomial exactly as a power series [closed]
I claim, for $\operatorname{artanh}(\rho) = \frac{1}{2} \ln\left(\frac{1+\rho}{1-\rho}\right)$, i.e., the inverse hyperbolic tangent function, the following holds approximately under assumptions given ...
- 191
0
votes
1
answer
86
views
Multi-variate Picard-Lindelöf? Convergence of analytic PDEs (w/ commutative partial derivatives & value at a base point)
I am looking for a theorem to give existence and uniqueness of solutions to PDEs of the following form.
Find an analytic $u : \mathbb{R}^n \to \mathbb{R}^m$ satisfying the equations $\partial_{x_i}u =...
1
vote
1
answer
75
views
Lower bound of $\frac{f(x)}{x^{n+1}}$
Let $f:[0,a]\to \Bbb{R}_{\geq 0}$ be real analytic, $a<1$. Furthermore, $f(0) = 0$ and $f$ is strictly increasing on $[0,a]$. Let $n\in \Bbb{N}$ be the smallest positive integer such that $f^{(n)}(...
- 171
2
votes
0
answers
82
views
Multivariate Taylor Series equals iterated univariate Taylor Series
I am asking myself wether the following fact is true:
Let $f(x_1,\ldots,x_k) \colon U \subseteq \mathbb{R}^n \to \mathbb{R}$ be a multivariate function.
Do the multivariate Taylor series $$MT_{f}(x_1,\...
- 31
0
votes
0
answers
136
views
Antiderivatives via Taylor series and the FT of Calculus
If $f$ is a real function on an interval $[a,b]$ such that
$f$ is computationally tractable on $[a,b]$: you can calculate $f(x)$ to $n$ bits of precision using an algorithm which is polynomial in $n$ ...
- 827
6
votes
2
answers
410
views
Taylor expansion theorem for Gateaux differentiable functions
I am having a hard time studying Gateaux derivatives (see https://en.wikipedia.org/wiki/Gateaux_derivative), it seems that every author mentions the concept but only as a cliffhanger to study Fréchet ...
1
vote
0
answers
66
views
Taylor series of incomplete elliptic integral of first kind
Say I have an incomplete elliptic integral of first kind of the form
$$F(\varphi(z), k(z))=\int_0^{\varphi(z)} \frac{d \theta}{\sqrt{1-k(z)^2 \sin ^2 \theta}}$$
where each arguments are function of ...
- 111
0
votes
1
answer
128
views
Small phase approximation
Does anyone known how to prove that if $|\phi_k (r)| \ll 1$ for all $r$ and all $k=1,...,n\,$, the following equation
$$ S=\left|\int_0^\infty A(r)e^{-i[\phi_0(r)+\sum_{k=1}^n \phi_k(r)]} dr \right|^2 ...
- 13
0
votes
0
answers
153
views
Do we have tetration uniqueness by $ A = \inf \sum_n a_n^2 $?
Let $f$ be a real analytic (on at least $|x|<2$) and real solution of the functional equation $f(0) = 1,f(x+1) = \exp(f(x))$. For the existence of such $f$, see here.
Then
$$
f(x) = \sum_n a_n x^n ;...
- 763
0
votes
1
answer
58
views
Integration algorithm and analytic property
This question is the continuation of the previous one.
In the article about the integration of analytical polynomial - time computable function $f(x)$ with the Taylor series $$f(x) = \sum_{n=0}^{\...
- 81
4
votes
1
answer
518
views
Taylor expansion of Stieltjes Transform
I'm trying to derive a very basic result stated in several books on random matrix theory (e.g. Terry Tao's book and Potters & Bouchaud's book).
Given a symmetric matrix $A \in \mathbb{R}^{N \times ...
- 204
1
vote
0
answers
35
views
Exponential-like function equivalent for the Dixonian Elliptics
Is there some exponential-like function that acts as partner for the intricate Dixonian Elliptics, in a similar way that the Exponential function acts as a partner for the trigonometric functions ?
3
votes
1
answer
212
views
Other expansion for positive Taylor expansion
I was thinking of the following problem. Let $f$ be a Taylor expansion and $a_k$ the associated coefficients,
$$\forall x\in\mathbb{R},~f(x)\triangleq\sum_{k=0}^\infty a_kx^k.$$
Let suppose that we ...
- 393
2
votes
2
answers
127
views
Is there a restriction on the structure of the set of points where all derivatives of a $C^\infty$ real function are 0? [duplicate]
Let $f$ be an infinitely differentiable real function and let $Z(f)$ denote the set of points on which all derivatives of $f$ vanish. It is not hard to describe an $f$ such that $Z(f)$ is any ...
- 481
4
votes
3
answers
698
views
What or where is the series expansion of the function $\ln\bigl(\frac{\tan x}{x}-1\bigr)$ or $\ln(\tan x-x)$ around $x=0$?
It is known that
\begin{equation*}
\tan x=\sum_{k=1}^{\infty}\frac{2^{2k}\bigl(2^{2k}-1\bigr)}{(2k)!}|B_{2k}|x^{2k-1}, \quad |x|<\frac{\pi}{2}
\end{equation*}
and
\begin{equation*}
\ln\tan x=\ln x+\...
- 1,101
8
votes
1
answer
495
views
$f(f(z)) = z , f(\exp(z)) = \exp(f(z)) $?
While talking about tetration with my friend the following idea (re)occured.
$$f(f(z)) = z ,\quad f(\exp(z)) = \exp(f(z)) \tag{A}\label{A}$$
or variations of it like the weaker
$$f(f(f(f(z)))) = z ,\...
- 763
3
votes
0
answers
58
views
Jet at a singular point or a submanifold
Let $M$ be a smooth manifold, $p\in M$ and $f\in C^\infty(M\setminus\{p\})$.
We will say that $f$ has a power-law singularity at $p$ of order $\eta$ if for every smooth immersion $\gamma:(-1,1)\to M$ ...
- 1,488
78
votes
7
answers
8k
views
Roots of truncations of $ e^x - 1$
During a talk I was at today, the speaker mentioned that if you truncate the Taylor series for $e^x - 1$, you'll get lots of roots with nonzero real part, even though the full Taylor series only has ...
- 2,284
5
votes
1
answer
353
views
Family of functions with prescribed derivatives
Suppose $f: \mathbb C \times (-1,1) \to \mathbb C$ is a smooth function that satisfies $f(0,t)=1$ for all $t\in (-1,1)$. Assume that for any $k\in \mathbb N$, any $z \in \mathbb C$ and any $t \in (-1,...
- 4,115
2
votes
1
answer
170
views
Is this a valid method of extending convergence of the Maclaurin series for $\frac{x}{x+1}$ from $|x|<1$ to $\Re(x)>-1$?
I originally asked this question on Math StackExchange a few months ago and no answers or even comments have yet been posted, so I'm asking this question again here on Math OverFlow.
This Math ...
- 1,126
0
votes
0
answers
78
views
Could the patterns in the roots of the Riemann-Liouville differintegral of $(s-1)\,L(s, \chi_{q,j})$ be explained?
This question aims to extend this question to (automorphic) Dirichlet L-functions.
Take: $$f(s,q,j) = (s-1)\,L(s, \chi_{q,j})$$
with $q$ the modulus and $j$ the index of a character $\chi$. A fast way ...
- 4,169
2
votes
1
answer
267
views
Fourier series of an arbitrary function of a cosine function
Is there a general expression for the Fourier series of the function $f(a\cos(\omega t))$ in terms of the derivatives of $f$? Obviously, the function can be expressed as a Maclaurin series $f(0)+af'(0)...
- 31
0
votes
1
answer
98
views
Estimating the bound of the integral over whole $\mathbb{R}$ of the Taylor remainder term?
Let $f : \mathbb{R} \to \mathbb{R}$ be a smooth function which has a smooth inverse and satisfies the estimate
\begin{equation}
\lvert f(x) \rvert \leq \lvert x \rvert.
\end{equation}
Also, let $d\mu$ ...
- 3,507
2
votes
3
answers
235
views
Computing $_2F_2(a,a,a+1,a+1,z)$ (hypergeometric function)
Trying to implement the derivative of the gamma incomplete function, I encountered the hypergeometric function $_2F_2(a,a,a+1,a+1; z=-x)$, where $x$ would always be a positive real (and thus $z$ a ...
- 686
3
votes
0
answers
109
views
Convex minorants to convex functions, given partial Taylor expansion and smoothness estimate
Let $V$ denote a strictly convex function (in arbitrary dimension) whose Hessian is $L$-Lipschitz. Given only this knowledge, and the values of $\left\{ V \left( x \right), \nabla V \left( x \right), \...
- 801
0
votes
1
answer
201
views
A holomorphic function in the open unit disk satisfying certain properties
Does there exist a function which is holomorphic in $|z|<1,$ continuous in $|z|\leq1$ and such that the series $\sum |a_n|$ is divergent, where $a_n$'s coefficients in the Taylor series expansion ...
- 165
0
votes
0
answers
72
views
Linear approximation of multivariate function of bounded second order partial derivatives
I have a question about linear approximation in the multivariate case.\
Let $f:B^d_r\to \mathbb{R}$ be a real-valued $C^2$-function defined on the $d$-dimensional ball of radius $r$ centered at the ...
- 93
3
votes
0
answers
176
views
What is the meaning of big-O of a random variable?
I encountered this problem in a book "Pattern Recognition and Machine Learning" by Christopher M. Bishop. I excerpt it below:
screenshot of the book
In the excerpt, the big-O notation $O(\xi^...
- 31
4
votes
1
answer
3k
views
Taylor series on a Riemannian manifold
I need some help for the following problem.
Let $M$ a riemannian manifold and $f$ a smooth differential function, then consider the following integral $$\int_M \Gamma(x,y)(f(y)-f(x))dV_y$$
where $dV_y$...
- 49
11
votes
1
answer
413
views
Estimating the growth of the Taylor coefficients given the growth of the function at the boundary
Let $f(z)=\sum a_nz^n$ be a Taylor series that converges for $|z|<1$ and satisfies
$$
|f(z)|\le \frac{1}{(1-|z|)^{k}}
$$
for some fixed $k>0$.
Question: What can I deduce about the growth of the ...
- 43.2k
1
vote
0
answers
160
views
Lower bound for variance of ratio of dependent random variables
I'm trying to find a lower bound on $\text{Var}(X / Y)$ for dependent random variables $X, Y \in [0, 1]$ with $X \leq Y$.
More specifically, $X$ and $Y$ are defined as follows:
Let $h, n \in \mathbb{N}...
- 299
3
votes
1
answer
210
views
Probabilistic Taylor theorem for concave functions
This paper proves a probabilistic version of Taylor's theorem
\begin{equation*}
\mathbb{E}g(X) = \sum_{k=0}^{n-1} \frac{g^{(k)}(0)}{k!} \mathbb{E}X^k + \frac{\mathbb{E}X^n}{n!} \mathbb{E} g^{(n)}(X_{(...
- 45
1
vote
0
answers
52
views
A problem on monotonicity rule for the ratio of two Maclaurin power series
In the paper [1] below, a monotonicity rule for the ratio of two Maclaurin power series was presented as follow.
Let $a_k$ and $b_k$ for $k\in\{0\}\cup\mathbb{N}$ be real numbers and
the power series ...
- 1,101
3
votes
1
answer
81
views
Exponential taylor series for multiple variables with linear constraints for coefficients
I'm trying to simplify the sum
$$
\sum_{\vec x \in (\mathbb{N}_0)^n: M\vec x = \vec b} \prod_i \frac{(a_i)^{x_i}}{x_i!},
$$
where $M$ is a $\mathbb{N}_0$-valued $m\times n$ matrix, $\vec b$ is $\...
- 3,001
8
votes
3
answers
759
views
Transformation converting power series to Bernoulli polynomial series
I wonder, can anyone describe an expression or formula of a transform that converts
$$\sum_{k=0}^\infty \frac{a_k x^k}{k!}$$
into
$$\sum_{k=0}^\infty \frac{a_k B_k(x)}{k!}$$
where $B_k(x)$ are ...
- 10.1k
8
votes
1
answer
2k
views
Polynomial approximation for square root function with fast convergence and bounded coefficients
Let $\delta, \varepsilon \in (0,1)$. I am interested in a sequence $\{f_n\}$ of polynomial approximations of the square root function $x \to x^{1/2}$ on $[\delta,1]$, of the form
$$
f_n(x) = \sum_{i=0}...
- 483
10
votes
2
answers
1k
views
Fourier series of $\log(a +b\cos(x))$?
By numerical computation it seems like, if $a_0 < a_1$:
$$
\begin{multline}
\log({a_0}^2 + {a_1}^2 + 2 a_0 a_1 \cos(\omega t)) = \log({a_0}^2 + {a_1}^2) \\
+ \frac{a_0}{a_1}\cos(\omega t)
- \frac{...
6
votes
2
answers
1k
views
Analyzing the decay rate of Taylor series coefficients when high-order derivatives are intractable
This could be a soft question. I am trying to show that the $n$-th Taylor series coefficient of a function is $O(n^{-5/2})$. However, because the function is a function composition of another function ...
- 212
2
votes
1
answer
136
views
Expressing a vector valued function in terms of its derivatives
Consider a function
$$
f:\mathbb{R}^n\rightarrow\mathbb{R}^m
$$
given by $m$ functions $f_i:\mathbb{R}^n\rightarrow \mathbb{R}$ that we can assume to be polynomials in $x_1,\dots,x_n$.
Does there ...
- 23
6
votes
2
answers
404
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?
- 61
5
votes
0
answers
681
views
The Basel problem revisited?
In the Basel problem, the $sinc$ function is considered at the Wikipedia page.
Let me try to make an alternative function definition:
$$f(x) = \prod_{n=1}^\infty \left ( 1+ \frac{x^3}{n^3} \right ) = \...
- 3,144
1
vote
0
answers
59
views
Showing bound $\|\nabla_t \dot{\tilde{x}}_Y(h, 0)\| \le L \|Y\|_{2, \infty}$ for smooth homotopies of geodesics
This question pertains to Lemma 3.5 of this article. Let $M$ be a smooth Riemannian manifold and $\gamma$ some geodesic with respect to the Levi-Civita connection $\nabla$. For any $C^2$ vector field $...
- 63
25
votes
4
answers
5k
views
Why is $ \frac{\pi^2}{12}=\ln(2)$ not true?
This question may sound ridiculous at first sight, but let me please show you all how I arrived at the aforementioned 'identity'.
Let us begin with (one of the many) equalities established by Euler:
...
- 4,829
6
votes
0
answers
2k
views
Do smooth cutoff functions analytically continue functions?
My goal is to prove (or disprove) that sufficiently smooth and quickly decaying cutoff functions being tacked on to a Taylor series correctly extend the radius of convergence to the analytic ...
- 1,740
24
votes
3
answers
16k
views
What's an example of a function whose Taylor series converges to the wrong thing?
Can anyone provide an example of a real-valued function f with a convergent Taylor series that converges to a function that is not equal to f (not even locally)?
- 461