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A109131
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Admirable numbers such that the subtracted divisor is a triangular number.
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3
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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
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OFFSET
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1,1
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LINKS
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EXAMPLE
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a(2)=24 because 1+2+3+4+8+12-6 = 24 and the subtracted divisor is triangular.
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MATHEMATICA
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triQ[n_] := IntegerQ @ Sqrt[8n + 1]; Select[Range[1000], MemberQ[Divisors[#], (d = (DivisorSigma[1, #] - 2#)/2)] && triQ[d] &] (* Amiram Eldar, Sep 21 2019 *)
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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