a(n) = (n + 1)*(20*n^2 + 19*n + 6)/6.

Terms

a(0) =1a(1) =15a(2) =62a(3) =162a(4) =335a(5) =601a(6) =980a(7) =1492a(8) =2157a(9) =2995a(10) =4026a(11) =5270a(12) =6747a(13) =8477a(14) =10480a(15) =12776a(16) =15385a(17) =18327a(18) =21622a(19) =25290a(20) =29351a(21) =33825a(22) =38732a(23) =44092a(24) =49925a(25) =56251a(26) =63090a(27) =70462a(28) =78387a(29) =86885