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%I #12 Aug 05 2023 04:12:33
%S 66,78,120,4095,491536,523776
%N Admirable triangular numbers.
%C No further term between 523776 and 4050045000. - _R. J. Mathar_, Feb 11 2008
%C a(7) > 10^20, if it exists. - _Amiram Eldar_, Aug 05 2023
%F A111592 INTERSECT A000217. - _R. J. Mathar_, Feb 11 2008
%e a(1) = 66 because 11*(11+1)/2 = 66 and 1+2+3+11+22+33-6 = 66.
%t 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 *)
%Y Cf. A000217, A111592.
%K nonn,more
%O 1,1
%A _Jason Earls_, Aug 11 2005