a(n) = binomial(n,3) + 6*binomial(n,4) + 15*binomial(n,5) + 15*binomial(n,6).

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =10a(5) =55a(6) =215a(7) =665a(8) =1736a(9) =3990a(10) =8310a(11) =16005a(12) =28930a(13) =49621a(14) =81445a(15) =128765a(16) =197120a(17) =293420a(18) =426156a(19) =605625a(20) =844170a(21) =1156435a(22) =1559635a(23) =2073841a(24) =2722280a(25) =3531650a(26) =4532450a(27) =5759325a(28) =7251426a(29) =9052785