a(n) = (2^n/n!) * Product_{k=0..n-1} (4*k - 3).

Terms

a(0) =1a(1) =-6a(2) =-6a(3) =-20a(4) =-90a(5) =-468a(6) =-2652a(7) =-15912a(8) =-99450a(9) =-640900a(10) =-4229940a(11) =-28455960a(12) =-194449060a(13) =-1346185800a(14) =-9423300600a(15) =-66591324240a(16) =-474463185210