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A125511 Octagonal numbers of the form C*(3C - 2) with 3C - 2 = k^2 and C a composite number. 1
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 (list; graph; refs; listen; history; text; internal format)
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-Fran├žois 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

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Last modified March 3 01:22 EST 2021. Contains 341756 sequences. (Running on oeis4.)