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A292557 a(n) is the smallest number k such that 2k - sigma(k) = 2^n. 0
3, 5, 22, 17, 250, 134, 262, 257, 6556, 4124, 10330, 8198, 91036, 19649, 65542, 65537, 1442716, 524294, 1363258, 4194332, 4411642, 16442342, 16866106, 22075325, 156791188, 536871032, 2160104368, 536870918, 1074187546, 2147483654, 4295862586, 19492545788 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Primes of the form 2^n+1, i.e., Fermat primes (A019734) are terms of this sequence.

For n > 32, a(n) > 2 * 10^10.

LINKS

Table of n, a(n) for n=1..32.

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, 2k - DivisorSigma[1, k]] != n, k++]; k, {n, 31}]

PROG

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

CROSSREFS

Cf. A019734, A033879, A125246, A125247, A125248, A191363, A275997.

Sequence in context: A153889 A186751 A280037 * A147442 A025093 A025112

Adjacent sequences:  A292554 A292555 A292556 * A292558 A292559 A292560

KEYWORD

nonn

AUTHOR

XU Pingya, Sep 19 2017

STATUS

approved

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Last modified February 21 02:19 EST 2018. Contains 299388 sequences. (Running on oeis4.)