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

Terms

a(0) =0a(1) =9a(2) =48a(3) =200a(4) =780a(5) =2989a(6) =11424a(7) =43740a(8) =167940a(9) =646624a(10) =2496120a(11) =9657674a(12) =37442132a(13) =145422645a(14) =565722688a(15) =2203961396a(16) =8597496564a(17) =33578000572a(18) =131282408360a(19) =513791607378