a(n) = Sum_{k=0..n} binomial(4*n+2,k).

Terms

a(0) =1a(1) =7a(2) =56a(3) =470a(4) =4048a(5) =35443a(6) =313912a(7) =2804012a(8) =25211936a(9) =227881004a(10) =2068564064a(11) =18844224462a(12) =172186125456