A115904 Numbers k such that k + sigma(k) is a triangular number.

7, 10, 66, 136, 145, 196, 198, 234, 270, 438, 495, 528, 592, 606, 855, 910, 1045, 1158, 1428, 2208, 2718, 2922, 3016, 3663, 3808, 3916, 4506, 4975, 5043, 5136, 5598, 6366, 6600, 7080, 7498, 7821, 8172, 8200, 8217, 8814, 9318, 9730, 9738

Offset: 1

Author

Giovanni Resta, Feb 06 2006

Keywords

• nonn

Examples

			234 + sigma(234) = 780 = T(39).
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A069066, A115907, A115909.

Programs

Mathematica

trnoQ[n_]:=IntegerQ[(Sqrt[1+8n]-1)/2]; Select[Range[10000], trnoQ[#+ DivisorSigma[ 1,#]]&] (* Harvey P. Dale, Apr 26 2014 *)

PARI

isok(n) = ispolygonal(n + sigma(n), 3); \\ Michel Marcus, Jan 25 2014