%I M4400 N1855 #33 Apr 26 2022 21:48:40
%S 1,7,31,127,73,23,8191,151,131071,524287,337,47,601,262657,233,
%T 2147483647,599479,71,223,79,13367,431,631,2351,4432676798593,103,
%U 6361,881,32377,179951,2305843009213693951,92737,145295143558111,193707721,10052678938039,228479,439,100801,581283643249112959,2687,2593,167
%N a(n) = least primitive factor of 2^(2n+1) - 1.
%C For n > 0, 2^(a(n)-2n-2) == 1 (mod a(n)), since 2^(a(n)-1) == 2^(2n+1) == 1 (mod a(n)). - _Thomas Ordowski_, Aug 11 2021
%C a(n) == 1 (mod 2n+1). - _Thomas Ordowski_, Aug 11 2021
%D J. Brillhart et al., Factorizations of b^n +- 1. Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 2nd edition, 1985; and later supplements.
%D M. Kraitchik, Recherches sur la Théorie des Nombres. Gauthiers-Villars, Paris, Vol. 1, 1924, Vol. 2, 1929, see Vol. 2, p. 84.
%D N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H Max Alekseyev, <a href="/A002184/b002184.txt">Table of n, a(n) for n = 0..602</a>
%H J. Brillhart et al., <a href="http://dx.doi.org/10.1090/conm/022">Factorizations of b^n +- 1</a>, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002.
%H S. S. Wagstaff, Jr., <a href="http://www.cerias.purdue.edu/homes/ssw/cun/index.html">The Cunningham Project</a>
%F a(n) = A112927(2n+1). - _Max Alekseyev_, Apr 26 2022
%Y Cf. A002588, A112927.
%K nonn
%O 0,2
%A _N. J. A. Sloane_
%E More terms from _Don Reble_, Nov 14 2006