OFFSET
1,1
LINKS
Colin Barker, Table of n, a(n) for n = 1..1000
Eric Weisstein's World of Mathematics, Dutch Windmill Graph
Eric Weisstein's World of Mathematics, Vertex-Induced Subgraph
Wikipedia, Friendship graph
Index entries for linear recurrences with constant coefficients, signature (6,-9,4).
FORMULA
a(n) = 4^n + 3*n.
From Colin Barker, May 21 2017: (Start)
G.f.: x*(7 - 20*x + 4*x^2) / ((1 - x)^2*(1 - 4*x)).
a(n) = 6*a(n-1) - 9*a(n-2) + 4*a(n-3) for n>3. (End)
E.g.f.: exp(x)*(exp(3*x) + 3*x) - 1. - Stefano Spezia, Aug 25 2022
MATHEMATICA
Table[4^n + 3 n, {n, 30}]
LinearRecurrence[{6, -9, 4}, {7, 22, 73}, 40] (* Harvey P. Dale, May 25 2019 *)
PROG
(PARI) Vec(x*(7 - 20*x + 4*x^2) / ((1 - x)^2*(1 - 4*x)) + O(x^30)) \\ Colin Barker, May 21 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Giovanni Resta, May 04 2017
STATUS
approved