A081856 Numbers n such that 2n-1 divides 2^n-1.

1, 2, 8, 128, 228, 648, 1352, 1908, 3240, 4608, 5220, 5976, 11448, 13160, 13920, 21528, 22050, 23760, 23940, 24840, 30960, 31284, 31584, 31968, 32768, 37224, 46092, 46512, 47268, 60480, 65664, 66528, 78540, 78600, 81728, 82800, 84312, 98406, 102672, 103968

Offset

1,2

Comments

Subsequence of odd terms begins: 1, 763425, 10888425, 40068105, 142086921, 191345625, 462784725, 468545025, 552451809; see A233415. - Charles R Greathouse IV, Dec 04 2013

Links

Alois P. Heinz, Table of n, a(n) for n = 1..1000

Maple

a:= proc(n) option remember; local k;
      if n=1 then 1 else for k from 1+a(n-1)
      while 2&^k mod(2*k-1)<>1 do od; k fi
    end:
seq(a(n), n=1..40);  # Alois P. Heinz, May 27 2016

Mathematica

terms = 100; Reap[For[n=1; k=1, k <= terms, n++, If[Divisible[2^n-1, 2n-1], Print[k, " ", n]; Sow[n]; k++]]][[2, 1]] (* Jean-François Alcover, Apr 06 2017 *)
Join[{1}, Select[Range[110000], PowerMod[2, #, 2*#-1]==1&]] (* Harvey P. Dale, Jan 19 2019 *)

Prog

(PARI) is(n)=Mod(2, 2*n-1)^n==1 \\ Charles R Greathouse IV, Dec 04 2013

Crossrefs

Sequence in context: A193481 A156497 A064205 * A038533 A139290 A152922

Adjacent sequences: A081853 A081854 A081855 * A081857 A081858 A081859

Keyword

nonn

Author

Benoit Cloitre, Apr 11 2003

Extensions

Three more terms from Michel Marcus, Dec 04 2013

Status

approved