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A292558 a(n) is the smallest number k such that sigma(k) - 2k = 2^n. 0
20, 12, 56, 550, 572, 108, 860, 952, 1232, 6328, 3708, 40540, 37072, 79288, 327260, 357112, 302000, 527296, 1764056, 6506512, 38559776, 21893248, 42257216, 167771740, 90798560, 469761208, 508198064, 490304800, 1353048560, 2951488480, 5067417200 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For n > 31, a(n) > 1.724 * 10^10.

LINKS

Table of n, a(n) for n=1..31.

EXAMPLE

sigma(20) - 2*20 = 2^1, a(1) = 20.

sigma(108) - 2*108 = 64 = 2^6, a(6) = 108.

MATHEMATICA

Table[k = 1; While[Log[2, DivisorSigma[1, k] - 2k] != n, k++]; k, {n, 30}]

PROG

(PARI) a(n) = my(k=1); while(sigma(k) - 2*k != 2^n, k++); k; \\ Michel Marcus, Sep 19 2017

CROSSREFS

Cf. A033880, A088831, A088832, A088833, A141547, A175989, A275996.

Sequence in context: A154043 A073868 A215028 * A040382 A223283 A076119

Adjacent sequences:  A292555 A292556 A292557 * A292559 A292560 A292561

KEYWORD

nonn

AUTHOR

XU Pingya, Sep 19 2017

STATUS

approved

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Last modified October 19 05:08 EDT 2018. Contains 316336 sequences. (Running on oeis4.)