A069065 Sigma(n)-phi(n) is a triangular number.

1, 6, 33, 49, 62, 65, 70, 72, 74, 77, 90, 126, 206, 210, 216, 297, 371, 427, 435, 465, 611, 626, 662, 665, 715, 731, 779, 784, 790, 846, 851, 854, 855, 899, 917, 1147, 1183, 1311, 1322, 1397, 1400, 1464, 1529, 1530, 1560, 1687, 1766, 1781, 1799, 1800, 1810

Offset

1,2

Links

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

Mathematica

tnQ[n_]:=IntegerQ[(Sqrt[1+8(DivisorSigma[1, n]-EulerPhi[n])]-1)/2]; Select[ Range[ 2000], tnQ] (* Harvey P. Dale, Jan 08 2014 *)

Crossrefs

Sequence in context: A132548 A140521 A171141 * A073343 A157872 A153127

Adjacent sequences: A069062 A069063 A069064 * A069066 A069067 A069068

Keyword

easy,nonn

Author

Benoit Cloitre, Apr 05 2002

Status

approved