a(n) = floor(binomial(n,5)/6).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =1a(7) =3a(8) =9a(9) =21a(10) =42a(11) =77a(12) =132a(13) =214a(14) =333a(15) =500a(16) =728a(17) =1031a(18) =1428a(19) =1938a(20) =2584a(21) =3391a(22) =4389a(23) =5608a(24) =7084a(25) =8855a(26) =10963a(27) =13455a(28) =16380a(29) =19792