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

Terms

a(0) =0a(1) =3a(2) =22a(3) =73a(4) =172a(5) =335a(6) =578a(7) =917a(8) =1368a(9) =1947a(10) =2670a(11) =3553a(12) =4612a(13) =5863a(14) =7322a(15) =9005a(16) =10928a(17) =13107a(18) =15558a(19) =18297a(20) =21340a(21) =24703a(22) =28402a(23) =32453a(24) =36872a(25) =41675a(26) =46878a(27) =52497a(28) =58548a(29) =65047