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A115909 Numbers n such that sigma(n) times n is a triangular number. 2
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

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

LINKS

Table of n, a(n) for n=1..41.

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 *)

CROSSREFS

Cf. A115904, A115910.

Sequence in context: A230216 A326081 A245392 * A254940 A196724 A056644

Adjacent sequences:  A115906 A115907 A115908 * A115910 A115911 A115912

KEYWORD

nonn

AUTHOR

Giovanni Resta, Feb 06 2006

STATUS

approved

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Last modified April 6 21:24 EDT 2020. Contains 333286 sequences. (Running on oeis4.)