The sum of the principal diagonals of an n X n spiral.

Terms

a(0) =0a(1) =1a(2) =10a(3) =25a(4) =56a(5) =101a(6) =170a(7) =261a(8) =384a(9) =537a(10) =730a(11) =961a(12) =1240a(13) =1565a(14) =1946a(15) =2381a(16) =2880a(17) =3441a(18) =4074a(19) =4777a(20) =5560a(21) =6421a(22) =7370a(23) =8405a(24) =9536a(25) =10761a(26) =12090a(27) =13521a(28) =15064a(29) =16717