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%I #28 Jan 19 2019 15:39:10
%S 1,2,8,128,228,648,1352,1908,3240,4608,5220,5976,11448,13160,13920,
%T 21528,22050,23760,23940,24840,30960,31284,31584,31968,32768,37224,
%U 46092,46512,47268,60480,65664,66528,78540,78600,81728,82800,84312,98406,102672,103968
%N Numbers n such that 2n-1 divides 2^n-1.
%C Subsequence of odd terms begins: 1, 763425, 10888425, 40068105, 142086921, 191345625, 462784725, 468545025, 552451809; see A233415. - _Charles R Greathouse IV_, Dec 04 2013
%H Alois P. Heinz, <a href="/A081856/b081856.txt">Table of n, a(n) for n = 1..1000</a>
%p a:= proc(n) option remember; local k;
%p if n=1 then 1 else for k from 1+a(n-1)
%p while 2&^k mod(2*k-1)<>1 do od; k fi
%p end:
%p seq(a(n), n=1..40); # _Alois P. Heinz_, May 27 2016
%t 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 Join[{1},Select[Range[110000],PowerMod[2,#,2*#-1]==1&]] (* _Harvey P. Dale_, Jan 19 2019 *)
%o (PARI) is(n)=Mod(2,2*n-1)^n==1 \\ _Charles R Greathouse IV_, Dec 04 2013
%K nonn
%O 1,2
%A _Benoit Cloitre_, Apr 11 2003
%E Three more terms from _Michel Marcus_, Dec 04 2013
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