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A109750
Admirable triangular numbers.
0
66, 78, 120, 4095, 491536, 523776
OFFSET
1,1
COMMENTS
No further term between 523776 and 4050045000. - R. J. Mathar, Feb 11 2008
a(7) > 10^20, if it exists. - Amiram Eldar, Aug 05 2023
FORMULA
A111592 INTERSECT A000217. - R. J. Mathar, Feb 11 2008
EXAMPLE
a(1) = 66 because 11*(11+1)/2 = 66 and 1+2+3+11+22+33-6 = 66.
MATHEMATICA
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 *)
CROSSREFS
Sequence in context: A121478 A330809 A128896 * A127654 A293175 A032485
KEYWORD
nonn,more
AUTHOR
Jason Earls, Aug 11 2005
STATUS
approved