Sum of all terms on the two principal diagonals of a 2n+1 X 2n+1 square spiral.

Terms

a(0) =1a(1) =25a(2) =101a(3) =261a(4) =537a(5) =961a(6) =1565a(7) =2381a(8) =3441a(9) =4777a(10) =6421a(11) =8405a(12) =10761a(13) =13521a(14) =16717a(15) =20381a(16) =24545a(17) =29241a(18) =34501a(19) =40357a(20) =46841a(21) =53985a(22) =61821a(23) =70381a(24) =79697a(25) =89801a(26) =100725a(27) =112501a(28) =125161a(29) =138737