a(n) = (1/4)*(n^2 - 2*n)^2 + (9/4)*(n^2 - 2*n) + 6.

Terms

a(0) =4a(1) =6a(2) =15a(3) =40a(4) =96a(5) =204a(6) =391a(7) =690a(8) =1140a(9) =1786a(10) =2679a(11) =3876a(12) =5440a(13) =7440a(14) =9951a(15) =13054a(16) =16836a(17) =21390a(18) =26815a(19) =33216a(20) =40704a(21) =49396a(22) =59415a(23) =70890a(24) =83956a(25) =98754a(26) =115431a(27) =134140a(28) =155040a(29) =178296