%N Successive differences of index of triangular numbers sum of two pentagonal numbers.
%C The behavior of this sequence is unexpectedly quite complex.
%H Peter Kagey, <a href="/A332986/b332986.txt">Table of n, a(n) for n = 1..2500</a>
%Y Cf. A000217 (triangular numbers), A000326 (pentagonal numbers), A332987 (sums of two nonzero pentagonal numbers), A333009 (analog for indices of square numbers).
%Y Cf. First differences of A332985.
%A _Olivier Gérard_, Mar 05 2020