OFFSET
0,1
COMMENTS
The three roots of x^3 - 2*x^2 - x - 2 are c1=2.65896708... = A348909+1, c2=-0.32948354... + 0.80225455...*i, and c3=-0.32948354... - 0.80225455...*i.
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 = 2, e2 = -1, and e3 = 2, giving us e1^3 - 3*e1*e2 + 3*e3 = 8 + 6 + 6 = 20, as expected.
LINKS
FORMULA
a(n) = 2*a(n-1) + a(n-2) + 2*a(n-3).
G.f.: (3 - 4 x - x^2)/(1 - 2 x - x^2 - 2 x^3).
2*a(n) = 7*b(n) - b(n+1) for b(n) = A077996(n).
EXAMPLE
For n=3, a(3) = (2.65896708...)^3 + (-0.32948354... + 0.80225455...*i)^3 + (-0.32948354... - 0.80225455...*i)^3 = 20.
MATHEMATICA
LinearRecurrence[{2, 1, 2}, {3, 2, 6}, 30]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Greg Dresden and Hanzhang Fang, Aug 12 2022
STATUS
approved