A019863 Decimal expansion of sin(3*Pi/10) (sine of 54 degrees, or cosine of 36 degrees).

8, 0, 9, 0, 1, 6, 9, 9, 4, 3, 7, 4, 9, 4, 7, 4, 2, 4, 1, 0, 2, 2, 9, 3, 4, 1, 7, 1, 8, 2, 8, 1, 9, 0, 5, 8, 8, 6, 0, 1, 5, 4, 5, 8, 9, 9, 0, 2, 8, 8, 1, 4, 3, 1, 0, 6, 7, 7, 2, 4, 3, 1, 1, 3, 5, 2, 6, 3, 0, 2, 3, 1, 4, 0, 9, 4, 5, 1, 2, 2, 4, 8, 5, 3, 6, 0, 3, 6, 0, 2, 0, 9, 4, 6, 9, 5, 5, 6, 8

Offset

0,1

Comments

Midsphere radius of regular icosahedron with unit edges.

Also half of the golden ratio (A001622). - Stanislav Sykora, Jan 30 2014

Andris Ambainis (see Aaronson link) observes that combining the results of Barak-Hardt-Haviv-Rao with Dinur-Steurer yields the maximal probability of winning n parallel repetitions of a classical CHSH game (see A201488) asymptotic to this constant to the power of n, an improvement on the naive probability of (3/4)^n. (All the random bits are received upfront but the players cannot communicate or share an entangled state.) - Charles R Greathouse IV, May 15 2014

This is the height h of the isosceles triangle in a regular pentagon, in length units of the circumscribing radius, formed by a side as base and two adjacent radii. h = sin(3*Pi/10) = cos(Pi/5) (radius 1 unit). - Wolfdieter Lang, Jan 08 2018

Also the limiting value(L) of "r" which is abscissa of the vertex of the parabola F(n)*x^2 - F(n+1)*x + F(n + 2)(where F(n)=A000045(n) are the Fibonacci numbers and n>0). - Burak Muslu, Feb 24 2021

Links

Stanislav Sykora, Table of n, a(n) for n = 0..2000

Scott Aaronson, The NEW Ten Most Annoying Questions in Quantum Computing (2014)

Boaz Barak, Moritz Hardt, Ishay Haviv, and Anup Rao, Rounding Parallel Repetitions of Unique Games (2008)

Irit Dinur and David Steurer, Analytical approach to parallel repetition, arXiv:1305.1979 [cs.CC], 2013-2014.

Michael Penn, a golden value of cosine., YouTube video, 2021.

Wikipedia, Exact trigonometric constants

Wikipedia, Platonic solid

Index entries for algebraic numbers, degree 2

Formula

Equals (1+sqrt(5))/4 = cos(Pi/5) = sin(3*Pi/10). - R. J. Mathar, Jun 18 2006

Equals 2F1(4/5,1/5;1/2;3/4) / 2 = A019827 + 1/2. - R. J. Mathar, Oct 27 2008

Equals A001622 / 2. - Stanislav Sykora, Jan 30 2014

phi / 2 = (i^(2/5) + i^(-2/5)) / 2 = i^(2/5) - (sin(Pi/5))*i = i^(-2/5) + (sin(Pi/5))*i = i^(2/5) - (cos(3*Pi/10))*i = i^(-2/5) + (cos(3*Pi/10))*i. - Jaroslav Krizek, Feb 03 2014

Equals 1/A134972. - R. J. Mathar, Jan 17 2021

Equals 2*A019836*A019872. - R. J. Mathar, Jan 17 2021

Equals (A094214 + 1)/2 or 1/(2*A094214). - Burak Muslu, Feb 24 2021

Equals hypergeom([-2/5, -3/5], [6/5], -1) = hypergeom([-1/5, 3/5], [6/5], 1) = hypergeom([1/5, -3/5], [4/5], 1). - Peter Bala, Mar 04 2022

Example

0.80901699437494742410229341718281905886015458990288143106772431135263...

Maple

convert(sin(3*Pi/10), radical); # W. Edwin Clark, May 24, 2023
Digits:=100; evalf((1+sqrt(5))/4); # Wesley Ivan Hurt, Mar 27 2014

Mathematica

RealDigits[(1 + Sqrt[5])/4, 10, 111] (* Robert G. Wilson v *)
RealDigits[Sin[54 Degree], 10, 120][[1]] (* Harvey P. Dale, Apr 21 2018 *)

Prog

(PARI) (1+sqrt(5))/4 \\ Charles R Greathouse IV, Jan 16 2012

Crossrefs

Cf. A001622, A019827.

Platonic solids midradii: A020765 (tetrahedron), A020761 (octahedron), A010503 (cube), A239798 (dodecahedron).

Sequence in context: A197617 A005076 A296182 * A350747 A243456 A246772

Adjacent sequences: A019860 A019861 A019862 * A019864 A019865 A019866

Keyword

nonn,cons,easy

Author

N. J. A. Sloane.

Status

approved