a(n) = floor( n*(n-1)*(n-2)*(n-3)/25 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =4a(6) =14a(7) =33a(8) =67a(9) =120a(10) =201a(11) =316a(12) =475a(13) =686a(14) =960a(15) =1310a(16) =1747a(17) =2284a(18) =2937a(19) =3720a(20) =4651a(21) =5745a(22) =7022a(23) =8500a(24) =10200a(25) =12144a(26) =14352a(27) =16848a(28) =19656a(29) =22800