a(n) = n! if n is even, otherwise 0 (from Taylor series for cos x).

Terms

a(0) =1a(1) =0a(2) =2a(3) =0a(4) =24a(5) =0a(6) =720a(7) =0a(8) =40320a(9) =0a(10) =3628800a(11) =0a(12) =479001600a(13) =0a(14) =87178291200a(15) =0a(17) =0a(19) =0a(21) =0a(23) =0a(25) =0