A115904 - OEIS

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

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OFFSET

1,1

LINKS

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

EXAMPLE

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

MATHEMATICA

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

PROG

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

CROSSREFS

Cf. A069066, A115907, A115909.

Sequence in context: A249952 A187999 A097187 * A136865 A350533 A351685

Adjacent sequences: A115901 A115902 A115903 * A115905 A115906 A115907

KEYWORD

nonn

AUTHOR

Giovanni Resta, Feb 06 2006

STATUS

approved