OFFSET
0,1
COMMENTS
a(n) is the sum of the n-th powers of the three roots of x^3 - x^2 - x - 3. These roots are c1 = 2.130395..., c2 = -0.5651977... - i*1.0434274..., and c3 = -0.5651977... + i*1.0434274..., and so a(n) = c1^n + c2^n + c3^n. The real parts of c2 and c3 are A273065.
a(n) can also be determined by Vieta's formulas and Newton's identities. For example, a(3) by definition is c1^3 + c2^3 + c3^3, and from Newton's identities this equals e1^3 - 3*e1*e2 + 3*e3 for e1, e2, e3 the elementary symmetric polynomials of x^3 - x^2 - x - 3. From Vieta's formulas we have e1 = 1, e2 = -1, and e3 = 3, giving us e1^3 - 3*e1*e2 + 3*e3 = 1 + 3 + 9 = 13, as expected.
LINKS
FORMULA
a(n) = a(n-1) + a(n-2) + 3*a(n-3) with a(0)=3, a(1)=1, a(2) = 3.
G.f.: (3 - 2*x - x^2)/(1 - x - x^2 - 3*x^3).
EXAMPLE
For n=3, a(3) = (2.130395...)^3 + (-0.5651977... - i*1.0434274...)^3 + (-0.5651977... + i*1.0434274...)^3 = 13.
MATHEMATICA
LinearRecurrence[{1, 1, 3}, {3, 1, 3}, 40]
PROG
(PARI) polsym(x^3 - x^2 - x - 3, 35) \\ Joerg Arndt, Aug 11 2022
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Greg Dresden, Aug 05 2022
STATUS
approved