a(n) = (2*n^3 - 6*n^2 + 19*n - 9)*n/6.

Terms

a(0) =1a(1) =7a(2) =24a(3) =66a(4) =155a(5) =321a(6) =602a(7) =1044a(8) =1701a(9) =2635a(10) =3916a(11) =5622a(12) =7839a(13) =10661a(14) =14190a(15) =18536a(16) =23817a(17) =30159a(18) =37696a(19) =46570a(20) =56931a(21) =68937a(22) =82754a(23) =98556a(24) =116525a(25) =136851a(26) =159732a(27) =185374a(28) =213991a(29) =245805