a(n) = n*(27*n + 1)/2.

Terms

a(0) =0a(1) =14a(2) =55a(3) =123a(4) =218a(5) =340a(6) =489a(7) =665a(8) =868a(9) =1098a(10) =1355a(11) =1639a(12) =1950a(13) =2288a(14) =2653a(15) =3045a(16) =3464a(17) =3910a(18) =4383a(19) =4883a(20) =5410a(21) =5964a(22) =6545a(23) =7153a(24) =7788a(25) =8450a(26) =9139a(27) =9855a(28) =10598a(29) =11368