A109750 Admirable triangular numbers.

66, 78, 120, 4095, 491536, 523776

Offset: 1

Author

Jason Earls, Aug 11 2005

Keywords

• nonn
• more

Comments

No further term between 523776 and 4050045000. - R. J. Mathar, Feb 11 2008

a(7) > 10^20, if it exists. - Amiram Eldar, Aug 05 2023

Examples

			a(1) = 66 because 11*(11+1)/2 = 66 and 1+2+3+11+22+33-6 = 66.
		

Crossrefs

Cf. A000217, A111592.

Programs

Mathematica

admQ[n_] := (ab = DivisorSigma[1, n] - 2 n) > 0 && EvenQ[ab] && ab/2 < n && Divisible[n, ab/2]; Select[Accumulate[Range[1024]], admQ] (* Amiram Eldar, Aug 05 2023 *)

Formula

A111592 INTERSECT A000217. - R. J. Mathar, Feb 11 2008