a(n) = 8*n^3 - 6*n - 1.

Terms

a(0) =-1a(1) =1a(2) =51a(3) =197a(4) =487a(5) =969a(6) =1691a(7) =2701a(8) =4047a(9) =5777a(10) =7939a(11) =10581a(12) =13751a(13) =17497a(14) =21867a(15) =26909a(16) =32671a(17) =39201a(18) =46547a(19) =54757a(20) =63879a(21) =73961a(22) =85051a(23) =97197a(24) =110447a(25) =124849a(26) =140451a(27) =157301a(28) =175447a(29) =194937