%I A005420 M2609
%S A005420 3,7,5,31,7,127,17,73,31,89,13,8191,127,151,257,131071,73,524287,41,
%T A005420 337,683,178481,241,1801,8191,262657,127,2089,331,2147483647,65537,
%U A005420 599479,131071,122921,109,616318177,524287,121369,61681,164511353,5419
%N A005420 Largest prime factor of 2^n - 1.
%D A005420 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A005420 J. Brillhart et al., Factorizations of b^n +- 1. Contemporary Mathematics, 
               Vol. 22, Amer. Math. Soc., Providence, RI, 2nd edition, 1985; and 
               later supplements.
%H A005420 T. D. Noe, <a href="b005420.txt">Table of n, a(n) for n=2..500</a> (derived 
               from Brillhart et al.)
%H A005420 J. Brillhart et al., <a href="http://www.ams.org/online_bks/conm22/">
               Factorizations of b^n +- 1</a>, Contemporary Mathematics, Vol. 22, 
               Amer. Math. Soc., Providence, RI, 3rd edition, 2002.
%H A005420 S. S. Wagstaff, Jr., <a href="http://www.cerias.purdue.edu/homes/ssw/
               cun/index.html">The Cunningham Project</a>
%H A005420 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               MersenneNumber.html">Mersenne Number</a>
%e A005420 2^6-1 = 63 = 3*21 = 9*7, so a(6) = 7.
%o A005420 (PARI) for(n=2,44, v=factor(2^n-1); print1(v[matsize(v)[1],1],","))
%Y A005420 Sequence in context: A112927 A064078 A048857 this_sequence A161818 A161509 
               A108974
%Y A005420 Adjacent sequences: A005417 A005418 A005419 this_sequence A005421 A005422 
               A005423
%K A005420 nonn,easy
%O A005420 2,1
%A A005420 N. J. A. Sloane (njas(AT)research.att.com).
%E A005420 Description corrected by Michael Somos, Feb 24, 2002
%E A005420 More terms from Rick L. Shepherd (rshepherd2(AT)hotmail.com), Aug 22 
               2002