This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified October 21 08:27 EDT 2018. Contains 316405 sequences. (Running on oeis4.)