A275374 Numbers k that divide sigma(k*(k+1)/2).

1, 2, 3, 7, 12, 15, 31, 56, 63, 127, 135, 168, 234, 240, 255, 260, 384, 504, 511, 720, 819, 896, 992, 1023, 1080, 1344, 1512, 1638, 2047, 2240, 2352, 3276, 3564, 3584, 3744, 3840, 4095, 4320, 4655, 7280, 7448, 8191, 9360, 10304, 12825, 12896, 13104, 14256, 14725, 15795, 16256

Offset: 1

Author

Altug Alkan, Jul 25 2016

Keywords

• nonn

Examples

			7 is a term because 7 divides sigma(28) = 56.
		

Crossrefs

Cf. A000203, A000217, A074285.

Programs

Mathematica

Select[Range@ 17000, Divisible[DivisorSigma[1, # (# + 1)/2], #] &] (* Michael De Vlieger, Jul 25 2016 *)

PARI

isok(n) = sigma(n*(n+1)/2) % n == 0