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

Terms

a(0) =0a(1) =1a(2) =4a(3) =18a(4) =80a(5) =350a(6) =1512a(7) =6468a(8) =27456a(9) =115830a(10) =486200a(11) =2032316a(12) =8465184a(13) =35154028a(14) =145608400a(15) =601749000a(16) =2481880320a(17) =10218366630a(18) =42004911960a(19) =172427570700a(20) =706905276000