%N Smallest k such that n, k and n+k have the same prime signature (canonical form), or 0 if no such number exists.
%C a(n) = 0 if n is a perfect prime power or a prime such that n+2 is composite. The zero entries in other cases suggest a generalization of Fermat's last theorem.
%C a(n) = 2 if n and n+2 form a twin prime pair.
%e a(12) = 63 as 12 + 63 = 75, 2^2*3 + 3^2*7 = 5^2*3, all have the prime signature p^2*q.
%A _Amarnath Murthy_ and Meenakshi Srikanth (menakan_s(AT)yahoo.com), Jul 01 2003
%E a(20)-a(47) from _Max Alekseyev_, Aug 12 2013