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 A109131 Admirable numbers such that the subtracted divisor is a triangular number. 10
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 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 Adjacent sequences:  A109128 A109129 A109130 * A109132 A109133 A109134 KEYWORD easy,nonn AUTHOR Jason Earls, Aug 17 2005 STATUS approved

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Last modified June 6 01:16 EDT 2020. Contains 334858 sequences. (Running on oeis4.)