Questions tagged [convergence-divergence]

Convergence and divergence of sequences and series and different modes of convergence and divergence. Also for convergence of improper integrals.

This tag is for questions about Convergent Sequences and Series and their existence, and by extension also for divergent ones. Also for convergence/divergence of improper integrals.

Formally, a sequence $S_n$ converges to the limit $S.$

$$\lim_{n\to \infty}S_n=S $$ if, for any $\epsilon>0$, there exists an $N>0$ such that $|S_n-S|<\epsilon$ for $n>N$.

A sequence diverges if it does not converge.

For more specialized questions about divergent series, such as summation methods, consider the tag .

21990 questions
0
votes
1 answer

Find the radius of convergence for complex series

[1]The series is $$\sum_{n=1}^\infty \frac{2^n(1-i)^\left(2n\right)}{(1+i)^\left(n+1\right)}*(z+i)^n$$. If i use the ratio rule it becomes$$\sum_{n=1}^\infty \frac{2^\left(n+1\right)(1-i)^\left(2n + 1\right)}{(1+i)^\left(n + 2\right)}*(z+i)^\left(n…
0
votes
2 answers

For what values of y does this series converge?

$\sum_{n=2}^{\inf} \frac{|y|^{1/n}}{nlog^2n}$ What could I use if I wanted to do a comparison test?
ramseys
  • 19
0
votes
1 answer

Question about a sufficient condition for a monotone (increasing) sequence to converge

Can I infer the convergence of a monotonically increasing sequence with first term a>0 using the fact that the ratio of consecutive terms tends to 1? Thanks in advance.
0
votes
1 answer

Convergence of $\sum_{n=1}^{\infty} (n^{n^\alpha}-1)$

I am trying to show for which parameter $\alpha$ does the sum$$\sum_{n=1}^{\infty} (n^{n^\alpha}-1)$$ converge. However I dont really know which criteria use.
mr.pink
  • 59
0
votes
0 answers

Alternating Double Series

Determine whether the series converges for $\frac{1}{2} < p \leq 1$ $$\sum_{k=2}^\infty \Bigl(\sum_{j=1}^{k-1} \frac{(-1)^k}{[j(k-j)]^p} \Bigr)$$ I've been stuck on this for a while now, and I'm not quite sure how to proceed with this. My assumption…
S10000
  • 369
  • 2
  • 5
0
votes
1 answer

Convergence of function (elementary)

For what values of $a$ does the function $f(x) = x^a e^{-x}$ converge as $x \rightarrow 0$? It turns out that the answer is $a<-1$ but I don't understand how to arrive at the answer. Any help is appreciated.
0
votes
1 answer

Radius and Interval of Convergence issue

What do I do when I end up with x as a square in an inequality? e.g. $-1 < (x+1)^2 < 1$ or $ -1 < 2(x+2)^2 <1$ ? Should there even be a negative interval since x is squared? How would we find the interval and radius of convergence when we end up…
0
votes
0 answers

"Trial and error" method to find a solution and covergence

Why is it that a "trial and error" method, choose a value of $x$ and use the right hand side of the equation to find a new value of $x$ and use that new vale of $x$ in the right hand side of the equation etc . . . . . . , to solve for $x$ does not…
Farcher
  • 166
0
votes
1 answer

Absolutely convergent vs conditionally convergent?

Why is the series $a_n={1\over2^n+n}$ convergent whereas the series $a_n={3^n+1\over4^n+5}$ is absolutely convergent? Since both use the comparison test to the geometric series, what makes one abs. convergent and the other not? I know a series…
0
votes
0 answers

Strong and weak convergence in $L^p((-1,+\infty))$ exercise

I can't solve this exercise: Consider the sequence of function $f_n:[1,+\infty\rangle > \to\mathbb{R},\;n\in\mathbb{N}$ $$f_n(x):=\frac{x-1}{\left(x+\frac{1}{n}\right)^{3/2}\ln(x+\frac{1}{n})}$$ Study the convergence properties of the sequence…
0
votes
2 answers

Series divergent

I have a problem with proof of the divergence of this series: $$\ \sum_{i=1}^\infty \frac{\sqrt {n+1} - \sqrt n }{\sqrt[3]n}$$ I got: $$\ \frac{1}{\sqrt[3]n(\sqrt {n+1} + \sqrt n)} \to 0$$ However, how can I prove that it is divergent? I suppose I…
0
votes
3 answers

Test the convergence of the sum $\sum_{n= 0 }^\infty\frac{4^{n-1}+2^n}{5^{n+1}}$

Does someone know how to evaluate the convergence of this series? I started by using the ratio test and setting $a_n = (4^{n-1}+2^n)/(5^{n+1})$, but after that step when rewriting I'm not really able to get rid of the $n$s. Would really appreciate…
Nickewas
  • 113
  • 8
0
votes
2 answers

Convergence in Newton's method

The given sequence comes from the recursion formula of Newton method $x_{n+1}=x_n-\frac{f(x_n)}{f'(x_n)}$ I am given $x_0=1, x_{n+1}=x_n-\frac{x^2_{n}-2}{2x_n}$ I need to show the value to which it converges . I have no idea how to proceed . Please…
0
votes
3 answers

Is the series convergent? Give a propf or counter-example

Let $$ \sum_{n=1}^\infty b_n$$ be a convergent series with $b_n > 0$ for all $n \geq 1$, and suppose p > 1. Is $$ \sum_{n=1}^\infty (b_n)^p$$ convergent? Justify your answer with a proof or give a counterexample. My guess is the if the series is…
0
votes
1 answer

Reduce power formula/ convergence /simplify

I can't seem to be able to simplify this $$x=(1-2*10^{-23})^{(2*10^{22})}$$ analytically, but it appears to converge to $x = \exp(-.4)$ (The exponent has an exponent.) Any help is deeply appreciated.