OFFSET
0,2
COMMENTS
2^n with gaps. In general, Sum_{k=0..n} Sum_{j=0..n} C(2(n-k),j)*C(2k,j)*r^j has expansion (1 - (r+1)*x)/((1 + (r+3)x + (r-1)(r+3)x^2 + (r-1)^3*x^3).
LINKS
FORMULA
G.f.: 1/(1-2x+4x^2-8x^3);
a(n) = 2*a(n-1) - 4*a(n-2) + 8*a(n-3);
a(n) = Sum_{k=0..n} Sum_{j=0..n} C(2(n-k), j)*C(2k, j)*(-1)^j.
a(n) = 2^n*A133872(n). - R. J. Mathar, Mar 08 2021
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Paul Barry, Jun 04 2005
STATUS
approved