a(n) = floor( n*(n-1)*(n-2)*(n-3)/26 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =4a(6) =13a(7) =32a(8) =64a(9) =116a(10) =193a(11) =304a(12) =456a(13) =660a(14) =924a(15) =1260a(16) =1680a(17) =2196a(18) =2824a(19) =3577a(20) =4472a(21) =5524a(22) =6752a(23) =8173a(24) =9808a(25) =11676a(26) =13800a(27) =16200a(28) =18900a(29) =21924