Central elements of the (1,2)-Pascal triangle A029635.

Terms

a(0) =1a(1) =3a(2) =9a(3) =30a(4) =105a(5) =378a(6) =1386a(7) =5148a(8) =19305a(9) =72930a(10) =277134a(11) =1058148a(12) =4056234a(13) =15600900a(14) =60174900a(15) =232676280a(16) =901620585a(17) =3500409330a(18) =13612702950a(19) =53017895700a(20) =206769793230a(21) =807386811660