a(n) = (4*n + 3)*(1 + 2*n^2)/3.

Terms

a(0) =1a(1) =7a(2) =33a(3) =95a(4) =209a(5) =391a(6) =657a(7) =1023a(8) =1505a(9) =2119a(10) =2881a(11) =3807a(12) =4913a(13) =6215a(14) =7729a(15) =9471a(16) =11457a(17) =13703a(18) =16225a(19) =19039a(20) =22161a(21) =25607a(22) =29393a(23) =33535a(24) =38049a(25) =42951a(26) =48257a(27) =53983a(28) =60145a(29) =66759