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

Terms

a(0) =1a(1) =3a(2) =17a(3) =39a(4) =81a(5) =139a(6) =225a(7) =335a(8) =481a(9) =659a(10) =881a(11) =1143a(12) =1457a(13) =1819a(14) =2241a(15) =2719a(16) =3265a(17) =3875a(18) =4561a(19) =5319a(20) =6161a(21) =7083a(22) =8097a(23) =9199a(24) =10401a(25) =11699a(26) =13105a(27) =14615a(28) =16241a(29) =17979