a(n) = A257851(n,n).

Terms

a(0) =1a(1) =9a(2) =27a(3) =80a(4) =216a(5) =448a(6) =1296a(7) =2816a(8) =6400a(9) =13312a(10) =30720a(11) =62208a(12) =139264a(13) =311296a(14) =688128a(15) =1474560a(16) =2985984a(17) =6029312a(18) =12845056a(19) =30408704a(20) =65011712a(21) =131072000a(22) =264241152a(23) =553648128a(24) =1132462080a(25) =2293235712a(26) =4697620480a(27) =9932111872a(28) =20132659200a(29) =41875931136