%I
%S 2,3,2,5,2,5,2,3,0,3,0,7,2,11,2,5,2,5,0,3,0,5,0,5,0,5,2,5,0,11,0,3,2,
%T 5,2,5,2,3,0,3,0,5,0,19,2,5,2,13,0,7,0,3,0,11,0,11,2,3,0,13,0,3,0,11,
%U 2,29,2,5,0,3,0,5,2,5,0,5,0,7,0,7,0,3,0,11,2,11,2,3,0,11,0,7,0,5,2,5,2,3,0,3
%N Least prime p such that n^2 + p^2 is prime, or 0 if none.
%C When n is odd, n^2 + p^2 is composite for all odd primes p, so a(n) = 2 or 0 according as n^2 + 2^2 is prime or not.
%C The locations of the zeros are in A263722.
%C The location of the first occurrence of prime(n) is A263466(n).
%e a(1) = 2 since 1^2 + 2^2 = 5 is prime.
%e a(2) = 3 since 2^2 + 2^2 = 8 is not prime but 2^2 + 3^2 = 13 is prime.
%e a(9) = 0 since 9^2 + 2^2 = 85 is not prime.
%t f[n_] := If[OddQ[n] && ! PrimeQ[n^2 + 4], 0,
%t Block[{p = 2}, While[! PrimeQ[n^2 + p^2] && p < 1500, p = NextPrime@p];
%t p]]; Array[f, 100]
%Y Cf. A240130, A240131, A263466, A263721, A263722, A263726, A263977.
%K nonn
%O 1,1
%A _Jonathan Sondow_ and _Robert G. Wilson v_, Nov 06 2015
