OFFSET
0,1
COMMENTS
Guide to related sequences:
sequence summand approximation minimal polynomial
(a(n)) sin(k*Pi/5)/2^k 0.6664488708 5 - 65*x^2 + 121*x^4
A373022 sin(2k*Pi/5)/2^k 0.5053526528 5 - 265*x^2 + 961*x^4
A373023 sin(3k*Pi/5)/2^k 0.3050180080 5 - 65*x^2 + 121*x^4
A373024 sin(4k*Pi/5)/2^k 0.1427344344 5 - 265*x^2 + 961*x^4
A373025 cos(k*Pi/5)/2^k 1.3503729060 11 - 23*x + 11*x^2
A373026 cos(2k*Pi/5)/2^k 0.8985194182 19 - 49*x + 31*x^2
A373027 cos(3k*Pi/5)/2^k 0.7405361848 11 - 23*x + 11*x^2
A373028 cos(4k*Pi/5)/2^k 0.6821257430 19 - 49*x + 31*x^2
FORMULA
Equals sqrt(10 - 2*sqrt*(5)) / (-8 + 2*sqrt(5)).
Equals (-1)*Sum_{k>=0} sin(9*k*Pi/5)/2^k.
Peter J. C. Moses (May 22 2024) found the following generalized summation identities for the eight sequences in Comments and many other sequences:
Sum_{k>=0} sin(h*k + Pi/m)/b^(k+r) = b^(1-r)*(b*sin(Pi/m) + sin(h - Pi/m)/(1 + b^2 - 2*b*cos*(Pi/m)).
Sum_{k>=0} cos(h*k + Pi/m)/b^(k+r) = b^(1-r)*(b*cos(Pi/m) + cos(h - Pi/m)/(1 + b^2 - 2*b*cos*(Pi/m)).
EXAMPLE
0.666448870812313914861635732850178653200791742032...
MATHEMATICA
{b, m, h} = {2, 5, 1}; s = Sum[Sin[ h k Pi/m]/b^k, {k, 0, Infinity}]
d = N[s, 100]
First[RealDigits[d], 100]
CROSSREFS
KEYWORD
nonn,cons
AUTHOR
Clark Kimberling, Jun 09 2024
STATUS
approved