login
A272536
Decimal expansion of the edge length of a regular 20-gon with unit circumradius.
4
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, 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, 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
OFFSET
0,1
COMMENTS
Since 20-gon is constructible (see A003401), this is a constructible number.
LINKS
Mauro Fiorentini, Construibili (numeri)
Eric Weisstein's World of Mathematics, Constructible Number
Wikipedia, Regular polygon
FORMULA
Equals 2*sin(Pi/20) = 2*A019818.
Equals also (sqrt(2)+sqrt(10)-2*sqrt(5-sqrt(5)))/4.
Equals i^(9/10) + i^(-9/10). - Gary W. Adamson, Jul 08 2022
EXAMPLE
0.3128689300804617380202106389343337846277997841713215801692826921...
MATHEMATICA
RealDigits[N[2Sin[Pi/20], 100]][[1]] (* Robert Price, May 02 2016*)
PROG
(PARI) 2*sin(Pi/20)
CROSSREFS
Cf. A003401.
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).
Cf. A019818.
Sequence in context: A204128 A266272 A201677 * A204122 A201657 A279384
KEYWORD
nonn,cons,easy
AUTHOR
Stanislav Sykora, May 02 2016
STATUS
approved