a(n) = floor( (2n-1)!!/(2n) ).

Terms

a(0) =0a(1) =0a(2) =2a(3) =13a(4) =94a(5) =866a(6) =9652a(7) =126689a(8) =1914412a(9) =32736453a(10) =624968662a(11) =13176422634a(12) =304071291562