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A115909
Numbers k such that sigma(k)*k is a triangular number.
3
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
OFFSET
1,2
COMMENTS
Infinite, since all the powers of 2 belong to the sequence.
LINKS
EXAMPLE
sigma(1421)*1421 = 2429910 = T(2204).
MATHEMATICA
Select[Range[540000], IntegerQ[(Sqrt[1+8(#*DivisorSigma[1, #])]-1)/2]&] (* Harvey P. Dale, Dec 16 2011 *)
PROG
(PARI) isok(n) = ispolygonal(n*sigma(n), 3); \\ Amiram Eldar, Apr 06 2023
CROSSREFS
KEYWORD
nonn
AUTHOR
Giovanni Resta, Feb 06 2006
STATUS
approved