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

%I A081856 #32 Sep 04 2024 12:59:55
%S A081856 1,2,8,128,228,648,1352,1908,3240,4608,5220,5976,11448,13160,13920,
%T A081856 21528,22050,23760,23940,24840,30960,31284,31584,31968,32768,37224,
%U A081856 46092,46512,47268,60480,65664,66528,78540,78600,81728,82800,84312,98406,102672,103968
%N A081856 Numbers k such that 2k-1 divides 2^k-1.
%C A081856 Subsequence of odd terms is given by A233415. - _Charles R Greathouse IV_, Dec 04 2013
%C A081856 Numbers 2k-1 form a subsequence of A187787. - _Max Alekseyev_, Sep 04 2024
%H A081856 Alois P. Heinz, <a href="/A081856/b081856.txt">Table of n, a(n) for n = 1..1000</a>
%p A081856 a:= proc(n) option remember; local k;
%p A081856       if n=1 then 1 else for k from 1+a(n-1)
%p A081856       while 2&^k mod(2*k-1)<>1 do od; k fi
%p A081856     end:
%p A081856 seq(a(n), n=1..40);  # _Alois P. Heinz_, May 27 2016
%t A081856 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 *)
%t A081856 Join[{1},Select[Range[110000],PowerMod[2,#,2*#-1]==1&]] (* _Harvey P. Dale_, Jan 19 2019 *)
%o A081856 (PARI) is(n)=Mod(2,2*n-1)^n==1 \\ _Charles R Greathouse IV_, Dec 04 2013
%Y A081856 Cf. A187787, A233415.
%K A081856 nonn
%O A081856 1,2
%A A081856 _Benoit Cloitre_, Apr 11 2003
%E A081856 a(38)-a(40) from _Michel Marcus_, Dec 04 2013