OFFSET
0,2
COMMENTS
Diagonal sums of number triangle A067804. In general, a(n)=sum{k=0..floor(n/2), C(2k,k)C(2(n-2k),n-2k)*r^k} has g.f. 1/sqrt(1-4x-4r*x^2+16r*x^3).
FORMULA
a(n)=sum{k=0..floor(n/2), C(2k, k)C(2(n-2k), n-2k)}.
D-finite with recurrence: n*a(n) +2*(1-2*n)*a(n-1) +4*(1-n)*a(n-2) +8*(2*n-3)*a(n-3)=0. - R. J. Mathar, Nov 09 2012
a(n) ~ 2^(2*n+1) / sqrt(3*Pi*n). - Vaclav Kotesovec, Feb 03 2014
MATHEMATICA
CoefficientList[Series[1/Sqrt[1-4*x-4*x^2+16*x^3], {x, 0, 20}], x] (* Vaclav Kotesovec, Feb 03 2014 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Paul Barry, Apr 24 2005
STATUS
approved