26,1

a(n) = 1 if and only if n is a triangular number.

Indices of conjectured 0's: 4, 9, 16, 25, 49, 64, 81, 121, 144, 169, 225, 256, 289, 361, 400, 441, 529, 576, 625, 729, ... These are squares of 2, 3, 4, 5, 7, 8, 9, 11, 12, 13, 15, 16, 17, 19, 20, 21, 23, 24, 25, 27.

Table of n, a(n) for n=26..48.

a(614) = 13964154294535688630985 = A000217(167117648945) because 614 * a(614) = A000217(4141012131555), and none of the smaller triangular numbers t satisfies 614*t = A000217(m) for some m.

Cf. A166478 (indices of t in A000217), A225502, A225503.

Sequence in context: A334248 A193256 A319038 * A340214 A088278 A242199

Adjacent sequences: A227051 A227052 A227053 * A227055 A227056 A227057

nonn

Alex Ratushnyak, Jun 29 2013

approved