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A225861 Numbers n such that (n + prime(n))/2 is a prime. 2

%I #11 May 31 2013 03:41:37

%S 15,19,21,23,25,31,37,39,57,67,75,77,83,99,107,111,115,145,165,175,

%T 189,195,207,235,237,247,273,293,303,311,315,337,351,353,361,395,405,

%U 439,445,471,477,493,495,499,537,545,555,567,569,581,585,591,619,631

%N Numbers n such that (n + prime(n))/2 is a prime.

%C Average of n and n-th prime is a prime. All n must be odd.

%H Paolo P. Lava, <a href="/A225861/b225861.txt">Table of n, a(n) for n = 1..1000</a>

%e 15 is in the list since (15+Prime(15))/2=31.

%t Select[Range[632], PrimeQ[(# + Prime[#])/2] &]

%Y Cf. A000040.

%K nonn

%O 1,1

%A _Jayanta Basu_, May 18 2013

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Last modified March 28 10:55 EDT 2024. Contains 371241 sequences. (Running on oeis4.)