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A115909
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Numbers k such that sigma(k)*k is a triangular number.
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3
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1, 2, 4, 8, 16, 32, 57, 64, 65, 85, 87, 128, 256, 512, 1024, 1245, 1421, 2048, 3146, 3330, 3790, 4096, 6695, 7257, 8192, 10137, 16384, 25884, 32768, 34420, 34551, 34947, 65536, 131072, 208495, 262144, 348161, 440495, 524288, 530270, 534430
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OFFSET
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1,2
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COMMENTS
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Infinite, since all the powers of 2 belong to the sequence.
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LINKS
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EXAMPLE
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sigma(1421)*1421 = 2429910 = T(2204).
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MATHEMATICA
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Select[Range[540000], IntegerQ[(Sqrt[1+8(#*DivisorSigma[1, #])]-1)/2]&] (* Harvey P. Dale, Dec 16 2011 *)
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PROG
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(PARI) isok(n) = ispolygonal(n*sigma(n), 3); \\ Amiram Eldar, Apr 06 2023
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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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