OFFSET
0,1
COMMENTS
Let A = sin(2*Pi/7), B = sin(4*Pi/7), C = sin(8*Pi/7).
In general, for integer h, k let
X = 2*sqrt(7)*A^(h+k+1)/(B^h*C^k),
Y = 2*sqrt(7)*B^(h+k+1)/(C^h*A^k),
Z = 2*sqrt(7)*C^(h+k+1)/(A^h*B^k).
then X, Y, Z are the roots of a monic equation
t^3 + a*t^2 + b*t + c = 0
where a, b, c are integers and c = 1 or -1.
Then X^n + Y^n + Z^n , n = 0, 1, 2, ... is an integer sequence.
This sequence has (h,k) = (0,1).
LINKS
Colin Barker, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (0,49,-49).
FORMULA
a(n) = (2*sqrt(7)*A^2/C)^n + (2*sqrt(7)*B^2/A)^n + (2*sqrt(7)*C^2/B)^n, where A = sin(2*Pi/7), B = sin(4*Pi/7), C = sin(8*Pi/7).
a(n) = 49*a(n-2) - 49 a(n-3) for n>2.
G.f.: (3 - 49*x^2) / (1 - 49*x^2 + 49*x^3). - Colin Barker, Dec 09 2018
MATHEMATICA
LinearRecurrence[{0, 49, -49}, {3, 0, 98}, 50] (* Amiram Eldar, Dec 09 2018 *)
PROG
(PARI) Vec((3 - 49*x^2) / (1 - 49*x^2 + 49*x^3) + O(x^25)) \\ Colin Barker, Dec 09 2018
(PARI) polsym(x^3 - 49*x + 49, 25) \\ Joerg Arndt, Dec 17 2018
CROSSREFS
KEYWORD
sign,easy
AUTHOR
Kai Wang, Dec 09 2018
STATUS
approved