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Prove or Disprove: if $\lim\limits_{n \to \infty} (a_{2n}-a_n)=0,$ then $\lim\limits_{n \to \infty} a_n=0.$

I don't think that this is true. and I'm trying to think about counterexample, but couldn't figure out a mathematical form of the sequence that I thought about, its a sequence where certain terms that are multipliers of $2n$ and $n$ become more close to each other as $n$ grows.

any hints ?

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    $\begingroup$ What about the constant sequence $a_n=1$? $\endgroup$
    – Olivier Bégassat
    Commented Nov 16, 2015 at 10:23
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    $\begingroup$ Maybe also interesting: Does $a_n$ converge if $a_{2n}-a_n$ converges? In the moment of typing I don't know the answer. $\endgroup$
    – fweth
    Commented Nov 16, 2015 at 11:08
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    $\begingroup$ @fweth Let $a_n = \gcd(3,n)$. Then $a_{2n} = a_n$ for all $n$, but $a_n$ does not converge. $\endgroup$
    – Najib Idrissi
    Commented Nov 16, 2015 at 12:34

1 Answer 1

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Just let $a_n = 1$ for all $n$, for example.

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