This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A136115 #2 Mar 31 2012 13:48:25 %S A136115 0,0,0,7,0,0,23,0,0,48,0,22,82,47,0,125,26,0,22,37,71,238,0,0,26,166, %T A136115 0,52,207,147,117,99,87,572,72,67,63,357,57,110,416,51,917,82,47,1050, %U A136115 217,380,167,246,0,97,697,0,374,191,537,1672,152,112,136,380,215,2037,68 %N A136115 Index m of least pentagonal number P(m) > P(n) such that P(m)+P(n) is again a pentagonal number, 0 if no such m exists. %F A136115 a(n)=0 iff n is in A136112 iff A000326(n) is in A136113. %e A136115 a(1..3)=0 since P(1),P(2),P(3) cannot be written as difference of 2 other pentagonal numbers > 0. %e A136115 a(4)=7 since P(7)=70 is the least pentagonal number > P(4)=22 such that their sum is again a pentagonal number, P(8). %o A136115 (PARI) P(n)=n*(3*n-1)>>1 /* a.k.a. A000326 */ /* newline */ isPent(t)=P(sqrtint(t<<1\3)+1)==t /* newline */ for(i=1,99,for(j=i+1,(P(i)-1)\3,isPent(P(i)+P(j))&print1(j",")|next(2));print1(0",")) %Y A136115 Cf. A000326, A136112-A136118. %K A136115 nonn %O A136115 1,4 %A A136115 _M. F. Hasler_, Dec 15 2007