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%I #8 Aug 10 2015 04:37:53
%S 1,1,1,2,1,3,1,1,2,1,3,1,1,2,5,4,2,1,2,1,3,4,2,2,5,4,1,1,2,3,5,3,1,2,
%T 6,3,1,5,4,5,4,1,2,2,1,4,1,2,2,3,3,2,5,7,1,3,3,1,2,1,4,1,1,4,1,4,1,2,
%U 2,5,3,3,1,2,1,5,4,1,5,1,3,2,10,2,1,3,6,1,2,1,4,1,5,10,3,3,2,10,7
%N Least k > 0 such that k^2 + (prime(n)-k)^2 is a prime, or 0 if no such k exists.
%C It appears that any prime (and also any odd number > 1, see A260870) can be written as the sum of two positive integers such that the sum of their squares is prime. For an even number > 2 this is obviously not possible since k and 2n-k have the same parity and therefore the sum of their squares is even.
%o (PARI) A260869(n)=for(k=1,(n=prime(n))\2,isprime(k^2+(n-k)^2)&&return(k))
%K nonn
%O 1,4
%A _M. F. Hasler_, Aug 09 2015
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