%N Indices of pentagonal pyramidal numbers which are the sum of two other such numbers: k such that A002411(k) = A002411(i)+A002411(j) for some i,j>0.
%C The numbers themselves are listed in A172425. This answers a question posed in A133459.
%H Donovan Johnson, <a href="/A172437/b172437.txt">Table of n, a(n) for n = 1..160</a>
%e a(1)=42 because A002411(42) is the smallest term in that sequence which is the sum of two other (nonzero) terms of A002411.
%o (Pari) for(n=1,99999,for(m=1,n-1, isA002411(p(n)-p(m)) & !print1(n", ") & break)) /* needs isA002411() and p() */
%Y Cf. A133459, A172425.
%A _M. F. Hasler_, Nov 20 2010