a(n) = floor( n*(n-1)*(n-2)*(n-3)/23 ).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =5a(6) =15a(7) =36a(8) =73a(9) =131a(10) =219a(11) =344a(12) =516a(13) =746a(14) =1044a(15) =1424a(16) =1899a(17) =2483a(18) =3193a(19) =4044a(20) =5055a(21) =6245a(22) =7633a(23) =9240a(24) =11088a(25) =13200a(26) =15600a(27) =18313a(28) =21365a(29) =24783