a(n) = 2*binomial(n,2) + 3*binomial(n,3) + 4*binomial(n,4).

Terms

a(0) =0a(1) =0a(2) =2a(3) =9a(4) =28a(5) =70a(6) =150a(7) =287a(8) =504a(9) =828a(10) =1290a(11) =1925a(12) =2772a(13) =3874a(14) =5278a(15) =7035a(16) =9200a(17) =11832a(18) =14994a(19) =18753a(20) =23180a(21) =28350a(22) =34342a(23) =41239a(24) =49128a(25) =58100a(26) =68250a(27) =79677a(28) =92484a(29) =106778