a(n) = n*(n + 1)*(n^2 - 3*n + 6)/8.

Terms

a(0) =1a(1) =3a(2) =9a(3) =25a(4) =60a(5) =126a(6) =238a(7) =414a(8) =675a(9) =1045a(10) =1551a(11) =2223a(12) =3094a(13) =4200a(14) =5580a(15) =7276a(16) =9333a(17) =11799a(18) =14725a(19) =18165a(20) =22176a(21) =26818a(22) =32154a(23) =38250a(24) =45175a(25) =53001a(26) =61803a(27) =71659a(28) =82650a(29) =94860