A128234 - OEIS

%I #4 Nov 18 2020 17:55:34

%S 8,9,11,12,19,23,28,62,64,68,71,85,95,104,114,115,131,134,175,178,181,

%T 182,200,216,240,246,247,250,266,276,277,316,346,372,382,421,429,438,

%U 440,444,445,458,469,485,494,511,518,541,543,566,568,594,596,601,604

%N Values of n such that the average of p(n) and p(p(n)), where p(k) denotes the k-th prime, is prime.

%e 9 is in the sequence because p(9)=23, p(23)=83 and (23+83)/2 is prime.

%p a:=proc(n) if isprime((ithprime(n)+ithprime(ithprime(n)))/2)=true then n else fi end: seq(a(n),n=2..800);

%t Select[Range[700],PrimeQ[Mean[{Prime[#],Prime[Prime[#]]}]]&] (* _Harvey P. Dale_, Nov 18 2020 *)

%Y Cf. A128233.

%K nonn

%O 2,1

%A _Emeric Deutsch_, Feb 20 2007