A081856 Numbers k such that 2k-1 divides 2^k-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

Author

Benoit Cloitre, Apr 11 2003

Keywords

• nonn

Comments

Subsequence of odd terms is given by A233415. - Charles R Greathouse IV, Dec 04 2013

Numbers 2k-1 form a subsequence of A187787. - Max Alekseyev, Sep 04 2024

Links

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

Crossrefs

Cf. A187787, A233415.

Programs

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 *)

PARI

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

Extensions

a(38)-a(40) from Michel Marcus, Dec 04 2013