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

Terms

a(0) =1a(1) =3a(2) =22a(3) =176a(4) =1471a(5) =12616a(6) =110056a(7) =971712a(8) =8656937a(9) =77663192a(10) =700614760a(11) =6349125440a(12) =57754842117a(13) =527046644056