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A109131
Admirable numbers such that the subtracted divisor is a triangular number.
1
20, 24, 30, 42, 54, 66, 78, 84, 102, 104, 114, 138, 140, 174, 186, 222, 224, 246, 258, 282, 308, 318, 354, 364, 366, 402, 426, 438, 464, 474, 476, 498, 532, 534, 582, 606, 618, 642, 644, 650, 654, 678, 762, 786, 812, 822, 834, 868, 894, 906, 942, 945, 978
OFFSET
1,1
LINKS
EXAMPLE
a(2)=24 because 1+2+3+4+8+12-6 = 24 and the subtracted divisor is triangular.
MATHEMATICA
triQ[n_] := IntegerQ @ Sqrt[8n + 1]; Select[Range[1000], MemberQ[Divisors[#], (d = (DivisorSigma[1, #] - 2#)/2)] && triQ[d] &] (* Amiram Eldar, Sep 21 2019 *)
CROSSREFS
Cf. A000217 (triangular numbers), A111592 (admirable numbers).
Sequence in context: A174629 A221651 A108373 * A171696 A060461 A214177
KEYWORD
easy,nonn
AUTHOR
Jason Earls, Aug 17 2005
STATUS
approved