%I A000020
%S A000020 2,1,2,2,6,6,18,16,48,60,176,144,630,756,1800,2048,7710,7776,27594,
%T A000020 24000,84672,120032,356960,276480,1296000,1719900,
%U A000020 4202496,4741632,18407808,17820000,69273666,67108864,211016256
%N A000020 Number of primitive polynomials of degree n over GF(2).
%D A000020 E. R. Berlekamp, Algebraic Coding Theory, McGraw-Hill, NY, 1968, p. 84.
%D A000020 T. L. Booth, An analytical representation of signals in sequential networks, 
               pp. 301-3240 of Proceedings of the Symposium on Mathematical Theory 
               of Automata. New York, N.Y., 1962. Microwave Research Institute Symposia 
               Series, Vol. XII; Polytechnic Press of Polytechnic Inst. of Brooklyn, 
               Brooklyn, N.Y. 1963 xix+640 pp. See p. 303.
%D A000020 R. Church, Tables of irreducible polynomials for the first four prime 
               moduli, Annals Math., 36 (1935), 198-209.
%D A000020 W. W. Peterson and E. J. Weldon, Jr., Error-Correcting Codes. MIT Press, 
               Cambridge, MA, 2nd edition, 1972, p. 476.
%D A000020 M. P. Ristenblatt, Pseudo-Random Binary Coded Waveforms, pp. 274-314 
               of R. S. Berkowitz, editor, Modern Radar, Wiley, NY, 1965; see p. 
               296.
%H A000020 David W. Wilson, <a href="b000020.txt">Table of n, a(n) for n = 1..400</
               a>
%o A000020 (PARI) a(n)=if(n==1,2,eulerphi(2^n-1)/n) - Hauke Worpel (thebigh(AT)outgun.com), 
               Jun 10 2008
%Y A000020 Cf. A058947, A011260.
%Y A000020 Sequence in context: A067541 A054706 A081727 this_sequence A077014 A093655 
               A023140
%Y A000020 Adjacent sequences: A000017 A000018 A000019 this_sequence A000021 A000022 
               A000023
%K A000020 nonn,easy
%O A000020 1,1
%A A000020 N. J. A. Sloane (njas(AT)research.att.com).
%E A000020 The initial 2 should really be a 1. See A011260 for official version.