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 A081856 Numbers k such that 2k-1 divides 2^k-1. 6
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 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 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 Cf. A187787, A233415. Sequence in context: A193481 A156497 A064205 * A038533 A139290 A152922 Adjacent sequences: A081853 A081854 A081855 * A081857 A081858 A081859 KEYWORD nonn,changed AUTHOR Benoit Cloitre, Apr 11 2003 EXTENSIONS a(38)-a(40) from Michel Marcus, Dec 04 2013 STATUS approved

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Last modified September 13 22:05 EDT 2024. Contains 375910 sequences. (Running on oeis4.)