a(n) = 3*binomial(2*n, n-1).

Terms

a(0) =0a(1) =3a(2) =12a(3) =45a(4) =168a(5) =630a(6) =2376a(7) =9009a(8) =34320a(9) =131274a(10) =503880a(11) =1939938a(12) =7488432a(13) =28973100a(14) =112326480a(15) =436268025a(16) =1697168160a(17) =6611884290a(18) =25792489800a(19) =100734001830a(20) =393847225200