1,1

The index of a triangular number T is the integer n such that T = n*(n+1)/2.

Peter Kagey, Table of n, a(n) for n = 1..1000

Cf. A000217 (triangular numbers), A000326 (pentagonal numbers), A332987 (sums of two nonzero pentagonal numbers).

Sequence in context: A049866 A118355 A026632 * A026654 A104370 A310013

Adjacent sequences: A332982 A332983 A332984 * A332986 A332987 A332988

nonn,easy

Olivier Gérard, Mar 05 2020

approved