A125511 Octagonal numbers of the form C*(3C - 2) with 3C - 2 = k^2 and C a composite number.

96, 225, 1408, 3400, 9633, 12936, 22016, 43681, 53600, 78408, 93633, 130625, 152776, 205408, 308481, 350208, 446216, 696008, 854400, 943041, 1250656, 1493896, 1628033, 1923201, 2085000, 2439008, 3280256, 3774408, 4041441

Offset

1,1

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Maple

From R. J. Mathar, Apr 22 2010: (Start)
A000567 := proc(n) n*(3*n-2) ; end proc:
for n from 4 to 1600 do if not isprime(n) then if issqr(3*n-2) then printf("%d, ", A000567(n) ); end if; end if; end do: (End)

Mathematica

Reap[Do[If[CompositeQ[n], If[IntegerQ[Sqrt[3n-2]], Sow[n(3n-2)]]], {n, 4, 10000}]][[2, 1]] (* Jean-François Alcover, Aug 19 2020 *)

Crossrefs

Cf. A124106.

Sequence in context: A270435 A110231 A108609 * A124106 A304510 A268771

Adjacent sequences: A125508 A125509 A125510 * A125512 A125513 A125514

Keyword

nonn

Author

Giovanni Teofilatto, Jan 19 2007

Extensions

Extended beyond 22016 by R. J. Mathar, Apr 22 2010

Status

approved