a(n) = n*(13*n^2 - 7)/6.

Terms

a(0) =0a(1) =1a(2) =15a(3) =55a(4) =134a(5) =265a(6) =461a(7) =735a(8) =1100a(9) =1569a(10) =2155a(11) =2871a(12) =3730a(13) =4745a(14) =5929a(15) =7295a(16) =8856a(17) =10625a(18) =12615a(19) =14839a(20) =17310a(21) =20041a(22) =23045a(23) =26335a(24) =29924a(25) =33825a(26) =38051a(27) =42615a(28) =47530a(29) =52809