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Let $p_n$ be the nth prime number, $p_1=2,p_2=3,p_3=5,\ldots$

How to prove this series converges/diverges?

$\sum_{n=1}^\infty \cos{p_n}$

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    to converge, terms must go to zero, so the set of limit points of $p(n)\text{mod}2\pi$ must be contained in $\{\pi/2,3\pi/2\}$, which doesnt seem very likely...2011-03-27

1 Answers 1

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If it converges, then this disproves the twin prime conjecture, I believe.

If $\lim\ \cos p_n = 0$ and the twin prime conjecture were true, then we would have that

as $p_n$ runs through the lower twin prime (i.e. both $p_n$ and $p_n + 2$ are primes),

$0 = \lim\ \cos (p_n + 2) = \lim\ (\cos p_n \cos 2 + \sin p_n \sin 2) = \pm \sin 2$

In fact,

If $\lim\ \cos p_n = 0$, then for any odd integer $M$, we must have that $\lim\ \cos (M\times p_n) = 0$ (as $\cos Mx$ can be written as an odd polynomial in $\cos x$), which I guess, implies that $ \lim\ \cos (2n+1) = 0$

If I remember correctly there was a previous question which disproved this.

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    I think I got it. Thanks.2012-02-15