A109750 Admirable triangular numbers.

66, 78, 120, 4095, 491536, 523776

Offset

1,1

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

Links

Table of n, a(n) for n=1..6.

Formula

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

Example

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

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 *)

Crossrefs

Cf. A000217, A111592.

Sequence in context: A121478 A330809 A128896 * A127654 A293175 A032485

Adjacent sequences: A109747 A109748 A109749 * A109751 A109752 A109753

Keyword

nonn,more

Author

Jason Earls, Aug 11 2005

Status

approved