This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A000020 #34 Mar 24 2025 21:54:21
%S A000020 2,1,2,2,6,6,18,16,48,60,176,144,630,756,1800,2048,7710,7776,27594,
%T A000020 24000,84672,120032,356960,276480,1296000,1719900,4202496,4741632,
%U A000020 18407808,17820000,69273666,67108864,211016256,336849900,929275200,725594112,3697909056
%N A000020 Number of primitive polynomials of degree n over GF(2) (version 2).
%C A000020 The initial 2 should really be a 1. See A011260 for official version.
%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 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="/A000020/b000020.txt">Table of n, a(n) for n = 1..400</a>
%H A000020 R. Church, <a href="http://www.jstor.org/stable/1968675">Tables of irreducible polynomials for the first four prime moduli</a>, Annals Math., 36 (1935), 198-209.
%H A000020 S. V. Duzhin and D. V. Pasechnik, <a href="ftp://pdmi.ras.ru/pub/publicat/znsl/v421/p081.pdf">Groups acting on necklaces and sandpile groups</a>, 2014. See p. 92. - _N. J. A. Sloane_, Jun 30 2014
%t A000020 Table[If[n==1,2,EulerPhi[2^n-1]/n],{n,1,50}] (* _Vladimir Joseph Stephan Orlovsky_, Jan 24 2012 *)
%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 (with initial term 1).
%K A000020 nonn
%O A000020 1,1
%A A000020 _N. J. A. Sloane_