OFFSET
0,1
FORMULA
a(n) = 4^(n+1) - C(n+1), C(n): Catalan numbers A000108; G.f. c(x)*(4-c(x))/(1-4*x), where c(x) = g.f. for Catalan numbers.
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
STATUS
approved