OFFSET
1,2
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (2,1,-2).
FORMULA
For n>1 a(n) = 5*2^(n-2)-(1-(-1)^n)/2.
From Colin Barker, Jun 26 2013: (Start)
For n>4 a(n) = 2*a(n-1)+a(n-2)-2*a(n-3).
G.f.: -x*(x^3+2*x^2-3*x-1) / ((x-1)*(x+1)*(2*x-1)). (End)
MATHEMATICA
Join[{1}, LinearRecurrence[{2, 1, -2}, {5, 9, 20}, 40]] (* Harvey P. Dale, Feb 03 2015 *)
PROG
(PARI) a(n) = sum(k=0, n, binomial(n+(-1)^k, k)); \\ Michel Marcus, Dec 06 2013
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Benoit Cloitre, Oct 27 2003
EXTENSIONS
Two more terms from Michel Marcus, Dec 06 2013
STATUS
approved