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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.)