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A352989
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Composites k such that the k-th triangular number is divisible by the integer log of k.
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3
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8, 15, 16, 27, 44, 54, 72, 84, 90, 95, 105, 125, 143, 150, 180, 195, 231, 256, 264, 287, 288, 308, 315, 319, 328, 351, 390, 423, 440, 483, 495, 512, 528, 540, 558, 559, 560, 576, 588, 608, 624, 627, 645, 648, 650, 728, 800, 805, 819, 840, 855, 870, 884, 896, 897, 924, 935, 945, 960, 975, 987
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OFFSET
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1,1
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COMMENTS
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Contains all odd members of A046346, and in particular p^(p^k) if p is an odd prime and k >= 1.
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LINKS
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EXAMPLE
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a(5) = 44 = 2*2*11 is a term because it is composite and A000217(44) = 44*45/2 = 990 is divisible by 2+2+11 = 15.
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MAPLE
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filter:= proc(n) local t; not isprime(n) and (n*(n+1)/2/add(t[1]*t[2], t=ifactors(n)[2]))::integer end proc:
select(filter, [$4..1000]);
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MATHEMATICA
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Select[Range[1000], CompositeQ[#] && Divisible[#*(# + 1)/2, Plus @@ Times @@@ FactorInteger[#]] &] (* Amiram Eldar, Apr 13 2022 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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