a(n) = (2n^2 - n + 2) * (2n)! / ((n + 1) * (n + 2) * n!^2).

Terms

a(0) =1a(1) =4a(2) =17a(3) =70a(4) =282a(5) =1122a(6) =4433a(7) =17446a(8) =68510a(9) =268736a(10) =1053626a(11) =4130524a(12) =16195220a(13) =63517950a(14) =249214545a(15) =978228870a(16) =3841579830a(17) =15093382920a(18) =59329549950a(19) =233324644020a(20) =918016888620