If n is even, 2(n/2 + 1)! - 1; if n is odd, ((n + 1)/2 + 1)! - 1.

Terms

a(0) =1a(1) =3a(2) =5a(3) =11a(4) =23a(5) =47a(6) =119a(7) =239a(8) =719a(9) =1439a(10) =5039a(11) =10079a(12) =40319a(13) =80639a(14) =362879a(15) =725759a(16) =3628799a(17) =7257599a(18) =39916799a(19) =79833599a(20) =479001599a(21) =958003199a(22) =6227020799a(23) =12454041599a(24) =87178291199a(25) =174356582399