Number of parity alternating permutations of [n] avoiding descents from odd to even numbers.

Terms

a(0) =1a(1) =1a(2) =2a(3) =1a(4) =5a(5) =2a(6) =20a(7) =6a(8) =114a(9) =24a(10) =864a(11) =120a(12) =8280a(13) =720a(14) =96480a(15) =5040a(16) =1325520a(17) =40320a(18) =20966400a(19) =362880a(20) =374855040a(21) =3628800a(22) =7468070400a(23) =39916800a(24) =163938297600a(25) =479001600a(27) =6227020800a(29) =87178291200