Questions regarding the Taylor series expansion of univariate and multivariate functions, including coefficients and bounds on remainders. A special case is also known as the Maclaurin series.

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108 views

Taylor's Theorem Problem

This is from my engineering mathematics textbook. Is this version of taylor's theorem correct ? Successive Differentiation, Maclaurin's and Taylor's Expansion of Function $-147$ TAYLOR'S THEOREM ...
2
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0answers
27 views

Entire periodic $f(z)$ with more than 50 % of the derivatives $0$?

Im looking for a real-entire function $f(z)$ such that for any complex $z$ : $1) $$f(z+p) =f(z)$ With $p$ a nonzero real number. $2)$ $f(z)= 0 + a_1 z + a_2 z^2 + a_3 z^3 + ...$ where more than ...
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0answers
27 views

Looking for a family of entire functions.

Let $i$ be a positive integer and let $f_i(x)$ be the $i$ th real transcendental entire function that has the Taylor expansion $f_i (x) = a_{i_0} + a_{i_1} x + a_{i_2} x^2 + ...$ with all $a_{i_n}$ ...
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0answers
21 views

Basis for a general function of tr$X^k$

I have a function of a $4\times 4$-matrix variable $X$ which is a general function of $\left<X^k\right>$ where $\left<\cdot\right>$ denotes the trace. My question is this: is there a ...
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3answers
124 views

Prove that $\lim_{x\to\infty} f'(x) = 0$ [duplicate]

Let $f(x)$ be twice differentiable on $(0,\infty)$ and let $\lim_{x\to \infty} f(x) = L<\infty$ and $|f''(x)| \le M$ for some $M>0$. Prove that $\lim_{x \to \infty} f'(x) = 0$. I've tried to ...
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0answers
24 views

Lagrange Bürmann Inversion Series Example

I am trying to understand how one applies Lagrange Bürmann Inversion to solve an implicit equation in real variables(given that the equation satisfies the needed conditions). I have tried looking for ...
1
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1answer
36 views

Taylor's series and the function argument dimension

I've stumbled over an interesting question. In $\cos(x)$, $x$ is measured in, say, radians. When I expand cosine in Taylor's series, I have the terms with $x^3$, $x^5$ etc. so I am summing up ...
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1answer
31 views

Geometric interpretation of a Taylor series like identity

Johann Bernoulli published (something like) the following expression in his journal Acta Eruditorum. $\int_0^x f(t) dt = xf(x)-\frac{x^2}{2!}f'(x)+\frac{x^3}{3!}f''(x)-\frac{x^4}{4!}f'''(x)+...$ Is ...
1
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0answers
24 views

$X\sim\mathcal N(0,1)$, Why is $\Phi_X^{(j)}(0)=0$ for $j$ odd?

If $X\sim\mathcal N(0,1)$ Why is $\Phi_X^{(j)}(0)=0$ for $j$ odd ? ($\Phi_X^{(j)}(0):j^{th}$ derivative of the characteristic function of the r.v. $X$) We computed ...
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5answers
1k views

What's the Maclaurin Series of $f(x)=\frac{1}{(1-x)^2}$?

This function seemed to be pretty much straight forward, but my solution is incorrect. I have two questions: 1. Where did I make a mistake? 2. I learned that there are shortcuts for finding a series ...
2
votes
1answer
24 views

Approximating error using Taylors theorem

I have used a Maclaurin series for the function $f(x) = \cos(2x)$ and have successfully produced: $\dfrac{2^n cos(\frac{n\pi}2)x^n}{n!}$ Now I want to estimate the error in approximating $\cos(2x)$ ...
8
votes
1answer
224 views

Real-analytic periodic $f(z)$ that has more than 50 % of the derivatives positive?

Im looking for a real-analytic function $f(z)$ such that for any $z$ $1) $$f(z+p) =f(z)$ With $p$ a nonzero real number and where $z$ is close to , or onto the real line such that $z$ is in the ...
0
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3answers
30 views

Maclaurin series stuck at finding $L_n$

I need to develop Maclaurin serie of $f(x)=\frac{1}{(1-x)^2}$ I found all the derivative, and all the zero values for the derivatives. I come up with that : ...
5
votes
1answer
54 views

Let $f:[-1,1] \to \mathbb{R}$ be differentiable 3 times, prove $\exists M>0 \ , \ s.t \ f(x) \le Mx^2$

Let $f:[-1,1] \to \mathbb{R}$ be differentiable 3 times, let $f(0)=0$ and $f(x) \ge 0 \ \forall x \in [-1,1]$. Prove: $\exists M>0 \ , \ s.t \ f(x) \le Mx^2$. I separated the proofs to ...
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2answers
24 views

How to show MacLaurin series for $\frac{1}{1-x}$ converges using remainder term

$\dfrac{1}{1-x} = 1 + x + x^2 + x^3 + \cdots$ this is valid for $x$ between $-1$ and $1$ not including the endpoints. How can one show that the Lagrange remainder goes to zero as $n$ goes to ...
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votes
1answer
14 views

First Order Approximation Taylor Series

I have the taylor series $f(z)=f(x_0)+(x-x_0)f'(z)+1/2(x-x_0)^2f''(z) ...$ and I am told that "As a first order approximation," $x-x_0$ ~ $\frac{f(x)-f(x_0)}{f'(x_0)}$ assuming $f'(x_0) \neq 0$ I ...
2
votes
1answer
46 views

Finding Laurent series where given annulus is not in a singularity

I'm given a problem where I need to calculate the Laurent series of $f(z)$ inside the given annulus $$ f(z) = {1\over z^3(z-1)}; \quad 1 < |z| < 2 $$ From online resources(videos, notes) I ...
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1answer
51 views

Maclaurin series of (1+x)^(1/x)

how can i find the Maclaurin series of $f(x)=(1+x)^{1 \over x}$? $f(0)$ is not even defined, or should I define it as $f(0)=e$? I stopped at the first derivative as it gets terribly messy. thank ...
2
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1answer
23 views

Using Taylor's series in imporper integrals

Is it possible to simplify an improper integral using Taylor's series? How can I prove this procedure is correct? For example, take $$f(\alpha)=\int_0^{\infty} ...
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2answers
61 views

The simplest way to pow using only simple arithmetic

i want to get function $f(x, a) = x^a$, for both x and a - real numbers, that uses only + - * /. So only way I found is: get taylor series for $$x^a = \sum_{n = ...
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2answers
3k views

Taylor series expansion for $f(x)=\sqrt{x}$ for $a=1$

I seem to be stuck defining an alternating sequence of terms in this series because $f^{(0)}(x)=f(x)$ is positive, as well as $f'(x)$, but then every other term starting with $f''(x)$ is negative. How ...
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1answer
168 views

Series expansion of square root function

Could I please know how to expand: $$\sqrt{4-3x^2}$$ I simplified it to $\sqrt{1-\frac34x^2}$ but the question is if I let $x = x^2$, what will the coefficient of, say $x^5$ or $x^{10}$ be in the ...
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0answers
12 views

Taylors formula

I have a circle $K(a,\epsilon) \subset \Omega $ and for $ \parallel \Delta x \parallel \lt \epsilon $ we look at $ \Delta f = f(a+ \Delta x) - f(a) $ Now I look at the function $ F:[0,1]\rightarrow ...
2
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1answer
37 views

Taylor series $\ln(2+x)$ centered at $x=2$

Taylor series $\ln(2+x)$ centered at $x=2$. Is the correct result $$y=\ln \left(4\right)+\sum _{n=1}^{∞}\frac{\left(-1\right)^n}{4^{\left(2^{\Large n}\right)}}\cdot \frac{\left(x-2\right)^n}{n!}\ ?$$ ...
1
vote
2answers
32 views

Meaning of interval of convergence when approximating functions

Let's say I have a Taylor series approximation, $p(x)$, of a function $f(x)$ at $a$: $$ p(x)=\sum_{n=0}^\infty{\frac{f^{(n)}(a)}{n!}(x-a)^n} $$ And that this Taylor series has a radius of ...
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2answers
119 views

Proving $\forall x\in\mathbb R : \dfrac{e^x + e^{-x}}2 \le e^{\frac{x^2}{2}}$ with Cauchy's MVT

Prove for all $x\in\mathbb R$: $$\dfrac{e^x + e^{-x}}2 \le e^{\frac{x^2}{2}}$$ Mclauren expansion: $$e^x=1+x+\frac{x^2}{2}+\frac{x^3}{3!}+R_4(x)$$ ...
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0answers
30 views

Taylor expansions, inequalities and more

(Part A) I have to find the Taylor expansion of order 2 around (0,0) of $$f: \mathbb{R}^{2}\rightarrow \mathbb{R}$$ $$(x,y)x \mapsto f(x,y) = x\log (1+y)+sin(x+y) $$ Furthermore I have to prove if ...
2
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1answer
56 views

Finding $\zeta(4)$ by Taylor series

Is it possible to solve Zeta(4) function using something similar to the solution for zeta(2) as seen in this video? https://www.youtube.com/watch?v=mTPKyC3Udns
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0answers
20 views

solution check for approximating derivative using a Taylor expansion.

I'm wondering if there's in a mistake in either my reasoning or the given solution for the problem and was hoping to have someone double check this for me. The problem states: Let $g(2)=3$ , ...
2
votes
3answers
274 views

Why is domain of convergence of Taylor series of $\ln(x)$ about $x=1$ is $ (0,2)$?

I can understand the lower bound as $\ln(x)$ doesn't exist for $x<0$. But how is the upper bound $2$?
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1answer
16 views

taylor series please i need immediate help

Expand 1/z by Taylor series about a point z=1. what I have done really makes no sense because I have no idea about it. I can only think of 1/z=1/z-1+1 1/z=1/((z-1)*(1/z-1))
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3answers
63 views

When does the remainder term in the taylor series go to zero?

When does the remainder term in the taylor series go to zero? Theorem: Let $f\in C^{N+1}([\alpha,\beta])$ and $x,x_0\in(\alpha,\beta)$. Then ...
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4answers
78 views

If $\displaystyle \sum_{n=0}^{\infty}c_{n}x^{n}=0$, show that $c_n= 0$ for any $n$ [closed]

Suppose that $f(x)=\displaystyle\sum_{n=0}^{\infty}c_{n}x^{n}$ for all $x$ with the radius of convergence $R>0$. If $\displaystyle\sum_{n=0}^{\infty}c_{n}x^{n}=0$, show that $c_n=0$ for any $n$.
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3answers
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Is there a formula similar to $f(x+a) = e^{a\frac{d}{dx}}f(x)$ to express $f(\alpha\cdot x)$?

Using the Taylor expansion $$f(x+a) = \sum_{k=0}^\infty \frac{a^k}{k!}\frac{d^k }{dx^k}f(x)$$ one can formally express the sum as the linear operator $e^{a\frac{d}{dx}}$ to obtain $$f(x+a) = ...
0
votes
2answers
55 views

Taylor series of $\sin(x)$ converges uniformly on $[-\pi,\pi]$?

According to my notes, the Taylor series of $\sin(x)$ converges uniformly on $[-\pi,\pi]$. I know that the remainder term needs to converge uniformly to $0$ for this to be the case. But I really ...
1
vote
1answer
51 views

How can I find the Maclaurin series of $\frac{1}{1+x+x^2}$?

How can I find the Maclaurin series of $\frac{1}{1+x+x^2}$? At first, I found the Maclaurin series of $\frac{1}{1+x}$, which is $\sum_{n=0}^{\infty}(-1)^{n}x^{n}$ and simply replaced $x$ with $x^2 + x ...
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2answers
56 views

$f(x)=\begin{cases}e^{\frac{-1}{x^2}} & \text{ if } x\neq 0 \\ 0& \text{ if } x= 0\end{cases}$ is not equal to its Maclaurin Series

$f(x)=\begin{cases}e^{\frac{-1}{x^2}} & \text{ if } x\neq 0 \\ 0& \text{ if } x= 0\end{cases}$ is not equal to its Maclaurin Series, which is ...
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1answer
29 views

Expanding $\frac{1}{\sqrt{1-x^2}}$ through the expansion of $\frac{1}{\sqrt{1-x}}$ by binomial series

I heard that to expand $\frac{1}{\sqrt{1-x^2}}$, I have to expand $\frac{1}{\sqrt{1-x}}$ by binomial series and then just replace $x$ to $x^2$. Using binomial series, I found that ...
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4answers
128 views

Two ways to show that $\sin x -x +\frac {x^3}{3!}-\frac {x^5}{5!}< 0$

Show that: $\large \sin x -x +\frac {x^3}{3!}-\frac {x^5}{5!}< 0$ on: $0<x<\frac {\pi}2$ I tried to solve it in two ways and got a little stuck: One way is to use Cauchy's MVT, define $f,g$ ...
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3answers
31 views

How can I expand $\frac{1}{\sqrt{1-x^2}}$ by using the binomial series?

How can I expand $\frac{1}{\sqrt{1-x^2}}$ by using the binomial series? I know how to expand $\frac{1}{\sqrt{1-x}}$, but I have no idea how to expand $\frac{1}{\sqrt{1-x^2}}$. Simply differentiate ...
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1answer
40 views

Cubic MacLaurin $e^{x^2}$

Find the Cubic MacLurin expansion of e^{x^2}. First, I tried the sub $t=x^2$ and used the regular expansion for $e^t$. But that was wrong. Can I not do non-linear substituions? My calculations: ...
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0answers
43 views

Understanding Taylor's Theorem

In our real analysis course, our lecturer has given us the following theorem, which I don't quite understand. It's been given in an odd way, not similar to anything I've found in books or on the ...
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2answers
39 views

How to show that $\cos(x)=\sum\limits_{n=0}^\infty (-1)^n \frac {x^{2n}}{ (2n)!} $

We know that cos(x) is infinitely differentiable and the Lagrange remainder $\rightarrow 0$ for all $x$, so the Taylor series indeed produces the function. We also know that $\cos^{(4k)}(x)=\cos(x)$ ...
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1answer
16 views

How can I find the Taylor series of a function using the known Taylor series of a related function?

I am trying to calculate the Taylor series for the function: $$f(x) = {\frac 1 x}(1 - \cos\sqrt{x})$$ How do I do it, if I know the Taylor series for $\cos(x)$? $\cos x = {\Large \sum\limits_{k = ...
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2answers
51 views

compute taylor series about $x=0$ of $\arctan(e^x -1 )$

hello I am having some issue and need a little guidance with this taylor expansion $$f(x)=arctan(e^x -1)$$ the terms i should get are $x+\frac{x^2}{2}-\frac{x^3}{6}-\frac{11 x^4}{24}-\frac{5 ...
2
votes
3answers
80 views

Taylor series of $\sqrt{\ln\left(\frac{1}{x}\right)}$

I am trying to compute the Taylor series of: $\sqrt{\ln\left(\frac{1}{x}\right)}$ I have computed the derivatives and evaluated them in $x=1/e$ but I cannot find the formula for the sequence of the ...
1
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3answers
71 views

Taylor expansion at $x=0$ of $\ln(1/(1-x))$

Hello I am having some trouble with the taylor expansion of $$f(x)= \ln \frac1{1-x}$$ Would it be correct to treat the inner part as the following geometric series? ...
2
votes
2answers
50 views

Trying to solve a Taylor series problem

I have a Taylor series problem, well more precisely a Maclaurin series. I am trying to find convergence of: $f(x) = e^{x^3} + e^{{2x}^3}$ Okay here goes: $$f'(x) = 3xe^{x^3} + 6x e^{{2x}^3}$$ ...
1
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5answers
102 views

Taylor series for $\sinh1$

I am doing taylor series and I want to do it on $\sinh1$. is there a way to make this problem really simple before I begin? note: $\sinh x= \cfrac{e^x - e^{-x}}2$ Any ideas are really helpful ...
1
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0answers
9 views

approximate a function by linear combination of its asympototics with fractional argument

Suppose that $0\le a \le 1$ and $0<f(x), g(ax)<\infty$ for $x\ge 0$ and $f(x)\to g(x)$ when $x\to \infty$. And also $g(a x)<g(b x)$ if $a<b$. Question: Can we use $g(a_k x)$ $(0\le a_1\lt ...