[ n(n-1)(n-2)(n-3)/11 ].

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =2a(5) =10a(6) =32a(7) =76a(8) =152a(9) =274a(10) =458a(11) =720a(12) =1080a(13) =1560a(14) =2184a(15) =2978a(16) =3970a(17) =5192a(18) =6676a(19) =8456a(20) =10570a(21) =13058a(22) =15960a(23) =19320a(24) =23184a(25) =27600a(26) =32618a(27) =38290a(28) =44672a(29) =51820