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%I #18 Jul 08 2022 20:48:32
%S 3,1,2,8,6,8,9,3,0,0,8,0,4,6,1,7,3,8,0,2,0,2,1,0,6,3,8,9,3,4,3,3,3,7,
%T 8,4,6,2,7,7,9,9,7,8,4,1,7,1,3,2,1,5,8,0,1,6,9,2,8,2,6,9,2,1,1,5,5,1,
%U 7,5,8,6,6,1,1,2,4,7,1,5,8,6,7,3,3,9,1,7,4,5,3,5,3,6,9,7,3,7,6,7,5,0,2,8,0
%N Decimal expansion of the edge length of a regular 20-gon with unit circumradius.
%C Since 20-gon is constructible (see A003401), this is a constructible number.
%H Stanislav Sykora, <a href="/A272536/b272536.txt">Table of n, a(n) for n = 0..2000</a>
%H Mauro Fiorentini, <a href="http://www.bitman.name/math/article/264">Construibili (numeri)</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/ConstructibleNumber.html">Constructible Number</a>
%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Constructible_number">Constructible number</a>
%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Regular_polygon">Regular polygon</a>
%F Equals 2*sin(Pi/20) = 2*A019818.
%F Equals also (sqrt(2)+sqrt(10)-2*sqrt(5-sqrt(5)))/4.
%F Equals i^(9/10) + i^(-9/10). - _Gary W. Adamson_, Jul 08 2022
%e 0.3128689300804617380202106389343337846277997841713215801692826921...
%t RealDigits[N[2Sin[Pi/20], 100]][[1]] (* _Robert Price_, May 02 2016*)
%o (PARI) 2*sin(Pi/20)
%Y Cf. A003401.
%Y Edge lengths of other constructible m-gons: A002194 (m=3), A002193 (4), A182007 (5), A101464 (8), A094214 (10), A101263 (12), A272534 (15), A272535 (16), A228787 (17).
%Y Cf. A019818.
%K nonn,cons,easy
%O 0,1
%A _Stanislav Sykora_, May 02 2016
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