a(n) = (n-3)*(n-2)^2*(n-1)^2*n/24.

Terms

a(0) =0a(1) =0a(2) =0a(3) =6a(4) =60a(5) =300a(6) =1050a(7) =2940a(8) =7056a(9) =15120a(10) =29700a(11) =54450a(12) =94380a(13) =156156a(14) =248430a(15) =382200a(16) =571200a(17) =832320a(18) =1186056a(19) =1656990a(20) =2274300a(21) =3072300a(22) =4091010a(23) =5376756a(24) =6982800a(25) =8970000a(26) =11407500a(27) =14373450a(28) =17955756a(29) =22252860