OFFSET
1,3
COMMENTS
Regular icosahedron: A three-dimensional figure with 20 congruent equilateral triangle faces, 12 vertices, and 30 edges.
Shorter diagonal of golden rhombus with unit edge length. - Eric W. Weisstein, Dec 11 2018
The length of the shorter side of a golden rectangle inscribed in a unit circle. - Michal Paulovic, Sep 01 2022
The side length of a square inscribed within a golden ellipse with a unit semi-major axis. - Amiram Eldar, Oct 02 2022
LINKS
Muniru A Asiru, Table of n, a(n) for n = 1..1000
J. Brandts, S. Korotov, M. Krizek, and J. Solc, On nonobtuse simplicial partitions, Siam Rev. 51 (2) (2009) 317-335.
Dr. Math, Regular Polyhedra: Formulas.
Eric Weisstein's World of Mathematics, Golden Rhombus.
Eric Weisstein's World of Mathematics, Icosahedron.
Wikipedia, Icosahedron.
FORMULA
Equals sqrt(50-10*sqrt(5))/5.
Equals csc(2*Pi/5). - Eric W. Weisstein, Dec 11 2018
Equals 1/Im(e^(3*i*Pi/5)) = 1/Im(e^(3*i*Pi/5) - 1) = sqrt(2 - 2/sqrt(5)). - Karl V. Keller, Jr., Jun 11 2020
Equals 1/A019881. - R. J. Mathar, Jan 17 2021
From Antonio GraciĆ” Llorente, Mar 15 2024: (Start)
Equals Product_{k >= 1} ((10*k - 1)*(10*k + 1))/((10*k - 2)*(10*k + 2)).
Equals Product_{k >= 1} 1/(1 - 1/(25*(2*k - 1)^2)). (End)
EXAMPLE
1.051462224238267212051338169695753214570995864486683563057871046482422...
MAPLE
evalf[120](csc(2*Pi/5)); # Muniru A Asiru, Dec 11 2018
MATHEMATICA
RealDigits[Csc[2 Pi/5], 10, 110][[1]] (* Eric W. Weisstein, Dec 11 2018 *)
PROG
(Python)
from decimal import *
getcontext().prec = 110
c = Decimal.sqrt(2 - 2 / Decimal.sqrt(Decimal(5)))
print([int(i) for i in str(c) if i != '.'])
# Karl V. Keller, Jr., Jul 10 2020
(PARI) sqrt(50-10*sqrt(5))/5 \\ Charles R Greathouse IV, Jan 22 2024
CROSSREFS
KEYWORD
AUTHOR
Vladimir Joseph Stephan Orlovsky, Jul 09 2010
EXTENSIONS
Partially rewritten by Charles R Greathouse IV, Feb 02 2011
STATUS
approved