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Products of two triangular numbers each > 1.
2

%I #11 Jul 31 2024 09:35:59

%S 9,18,30,36,45,60,63,84,90,100,108,126,135,150,165,168,198,210,216,

%T 225,234,270,273,280,315,330,360,396,408,420,441,450,459,468,513,540,

%U 546,550,570,588,630,660,675,693,720,756,759,780,784,816,825,828,900,910

%N Products of two triangular numbers each > 1.

%p isA140089 := proc(n)

%p for d in numtheory[divisors](n) minus {1} do

%p if d^2 > n then

%p return false;

%p end if;

%p if isA000217(d) then

%p if isA000217(n/d) then

%p return true;

%p end if;

%p end if;

%p end do:

%p return false;

%p end proc:

%p for n from 1 to 1000 do

%p if isA140089(n) then

%p printf("%d,",n) ;

%p end if;

%p end do: # _R. J. Mathar_, Jul 31 2024

%Y Cf. A000217. Subsequence of A085780.

%K nonn

%O 1,1

%A _N. J. A. Sloane_, Jun 27 2008

%E More terms from _Rick L. Shepherd_ and _John W. Layman_, Jul 03 2008