a(n) = Sum_{k=0..n} binomial(4*n-3,k).

Terms

a(0) =1a(1) =2a(2) =16a(3) =130a(4) =1093a(5) =9402a(6) =82160a(7) =726206a(8) =6474541a(9) =58115146a(10) =524472448a(11) =4754293704a(12) =43257431931a(13) =394821713910