OFFSET
1,1
LINKS
Eric Weisstein's World of Mathematics, Independent Vertex Set
Eric Weisstein's World of Mathematics, Vertex Cover
Eric Weisstein's World of Mathematics, Web Graph
Index entries for linear recurrences with constant coefficients, signature (2,8,4).
FORMULA
G.f.: -2*x*(1 + 8*x + 6*x^2)/(-1 + 2*x + 8*x^2 + 4*x^3).
a(n) = 2*a(n-1) + 8*a(n-2) + 4*a(n-3).
MATHEMATICA
Table[RootSum[-4 - 8 # - 2 #^2 + #^3 &, #^n &], {n, 20}]
LinearRecurrence[{2, 8, 4}, {2, 20, 68}, 20]
CoefficientList[Series[-2 (1 + 8 x + 6 x^2)/(-1 + 2 x + 8 x^2 + 4 x^3), {x, 0, 20}], x] (* Eric W. Weisstein, Sep 27 2017 *)
PROG
(PARI) Vec(-2*x*(1+8*x+6*x^2)/(-1+2*x+8*x^2+4*x^3) + O(x^30)) \\ Michel Marcus, Sep 27 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Eric W. Weisstein, May 25 2017
STATUS
approved