A069066 Sigma(n)+phi(n) is a triangular number.

3, 5, 18, 38, 94, 114, 130, 168, 198, 202, 222, 238, 247, 314, 495, 518, 531, 612, 630, 662, 722, 754, 812, 914, 969, 1007, 1125, 1138, 1254, 1258, 1260, 1313, 1316, 1558, 1665, 1736, 1778, 1794, 1957, 2110, 2138, 2233, 2279, 2327, 2360, 2364, 2385, 2474

Offset: 1

Author

Benoit Cloitre, Apr 05 2002

Keywords

• nonn

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Programs

Mathematica

triangularQ[n_] := IntegerQ[ Sqrt[8n + 1] ]; Select[ Range[2500], triangularQ[ DivisorSigma[1, #] + EulerPhi[#]] &] (* Jean-François Alcover, Jul 17 2013 *)

PARI

is(n)=ispolygonal(sigma(n=factor(n))+eulerphi(n),3) \\ Charles R Greathouse IV, Nov 27 2013