a(n) = floor(binomial(n,4)/4).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =1a(6) =3a(7) =8a(8) =17a(9) =31a(10) =52a(11) =82a(12) =123a(13) =178a(14) =250a(15) =341a(16) =455a(17) =595a(18) =765a(19) =969a(20) =1211a(21) =1496a(22) =1828a(23) =2213a(24) =2656a(25) =3162a(26) =3737a(27) =4387a(28) =5118a(29) =5937