a(n) = binomial(n, floor(n/2)-1).

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =4a(5) =5a(6) =15a(7) =21a(8) =56a(9) =84a(10) =210a(11) =330a(12) =792a(13) =1287a(14) =3003a(15) =5005a(16) =11440a(17) =19448a(18) =43758a(19) =75582a(20) =167960a(21) =293930a(22) =646646a(23) =1144066a(24) =2496144a(25) =4457400a(26) =9657700a(27) =17383860a(28) =37442160a(29) =67863915