a(n) = floor(Gamma(3n/4)).

Terms

a(0) =1a(1) =0a(2) =1a(3) =2a(4) =4a(5) =11a(6) =35a(7) =120a(8) =453a(9) =1871a(10) =8376a(11) =40320a(12) =207358a(13) =1133278a(14) =6552134a(15) =39916800a(16) =255371835a(17) =1710542068a(18) =11964299312a(19) =87178291200a(20) =660355655453