A115909 - OEIS

A115909

Numbers k such that sigma(k)*k is a triangular number.

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

1,2

COMMENTS

Infinite, since all the powers of 2 belong to the sequence.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..100

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

Cf. A064987, A000217, A115904, A115910.

Sequence in context: A230216 A326081 A245392 * A254940 A374495 A196724

Adjacent sequences: A115906 A115907 A115908 * A115910 A115911 A115912

KEYWORD

nonn

AUTHOR

Giovanni Resta, Feb 06 2006

STATUS

approved