OFFSET
0,1
COMMENTS
This is the ratio of the radius r of the circumscribing circle of a regular pentagon and its side length s: r/s = 1/(2*sin(Pi/5)).
A quartic number of denominator 5 and minimal polynomial 5x^4 - 5x^2 + 1. - Charles R Greathouse IV, Mar 04 2018
Appears at Schur decomposition of A=[1 2; 2 3]. - Donghwi Park, Jun 20 2018
LINKS
Muniru A Asiru, Table of n, a(n) for n = 0..2000
Wikipedia, Schur decomposition.
FORMULA
EXAMPLE
r/s = 0.850650808352039932181540497063011072240401403764816881836740242377...
2*r/s = A121570.
MATHEMATICA
RealDigits[1/(2 Sin[Pi/5]), 10, 111][[1]] (* Robert G. Wilson v, Jul 15 2018 *)
PROG
(PARI) 1/(2*sin(Pi/5)) \\ Charles R Greathouse IV, Mar 04 2018
(PARI) sqrt((5+sqrt(5))/10) \\ Charles R Greathouse IV, Mar 04 2018
CROSSREFS
KEYWORD
AUTHOR
Wolfdieter Lang, Mar 01 2018
STATUS
approved