A transform of the double factorial numbers A001147.

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =7a(5) =14a(6) =39a(7) =97a(8) =308a(9) =897a(10) =3139a(11) =10304a(12) =38997a(13) =140893a(14) =570002a(15) =2230599a(16) =9567979a(17) =40091222a(18) =181203603a(19) =805962157a(20) =3819522284a(21) =17912075229a(22) =88646095447a(23) =435959031488