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A125511 Octagonal numbers of the form C*(3C - 2) with 3C - 2 = k^2 and C a composite number. 1

%I

%S 96,225,1408,3400,9633,12936,22016,43681,53600,78408,93633,130625,

%T 152776,205408,308481,350208,446216,696008,854400,943041,1250656,

%U 1493896,1628033,1923201,2085000,2439008,3280256,3774408,4041441

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

%H Robert Israel, <a href="/A125511/b125511.txt">Table of n, a(n) for n = 1..10000</a>

%p From _R. J. Mathar_, Apr 22 2010: (Start)

%p A000567 := proc(n) n*(3*n-2) ; end proc:

%p 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)

%Y Cf. A124106.

%K nonn

%O 1,1

%A _Giovanni Teofilatto_, Jan 19 2007

%E Extended beyond 22016 by _R. J. Mathar_, Apr 22 2010

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Last modified January 27 12:01 EST 2020. Contains 331295 sequences. (Running on oeis4.)