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

Terms

a(0) =1a(1) =5a(2) =25a(3) =77a(4) =177a(5) =341a(6) =585a(7) =925a(8) =1377a(9) =1957a(10) =2681a(11) =3565a(12) =4625a(13) =5877a(14) =7337a(15) =9021a(16) =10945a(17) =13125a(18) =15577a(19) =18317a(20) =21361a(21) =24725a(22) =28425a(23) =32477a(24) =36897a(25) =41701a(26) =46905a(27) =52525a(28) =58577a(29) =65077