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 A272489 #16 Aug 21 2023 12:31:11 %S A272489 5,6,3,4,6,5,1,1,3,6,8,2,8,5,9,3,9,5,4,2,2,8,3,5,8,3,0,6,9,3,2,3,3,7, %T A272489 9,8,0,7,1,5,5,5,7,9,7,9,4,6,5,3,3,7,4,3,6,6,2,1,6,0,6,1,2,1,7,5,6,9, %U A272489 7,5,9,7,0,3,8,0,5,8,3,3,6,2,4,6,9,3,5,2,3,6,9,0,3,7,7,3,0,9,9,9,3,5,9,8,8 %N A272489 Decimal expansion of the edge length of a regular 11-gon with unit circumradius. %C A272489 The edge length e(m) of a regular m-gon is e(m) = 2*sin(Pi/m). In this case, m = 11, and the constant, a = e(11), is not constructible using a compass and a straightedge (see A004169). With an odd m, in fact, e(m) would be constructible only if m were a Fermat prime (A019434). %H A272489 Stanislav Sykora, <a href="/A272489/b272489.txt">Table of n, a(n) for n = 0..2000</a> %H A272489 Wikipedia, <a href="http://en.wikipedia.org/wiki/Constructible_number">Constructible number</a> %H A272489 Wikipedia, <a href="http://en.wikipedia.org/wiki/Regular_polygon">Regular polygon</a> %H A272489 <a href="/index/Al#algebraic_10">Index entries for algebraic numbers, degree 10</a> %F A272489 Equals 2*sin(Pi/11) = 2*cos(Pi*9/22). %e A272489 0.5634651136828593954228358306932337980715557979465337436621606121... %t A272489 RealDigits[N[2Sin[Pi/11], 100]][[1]] (* _Robert Price_, May 01 2016 *) %o A272489 (PARI) 2*sin(Pi/11) %Y A272489 Cf. A004169, A019434. %Y A272489 Edge lengths of nonconstructible n-gons: A272487 (n=7), A272488 (n=9), A272490 (n=13), A255241 (n=14), A130880 (n=18), A272491 (n=19). %K A272489 nonn,cons,easy %O A272489 0,1 %A A272489 _Stanislav Sykora_, May 01 2016