[ (2n-1)!!/(4n) ].

Terms

a(0) =0a(1) =0a(2) =1a(3) =6a(4) =47a(5) =433a(6) =4826a(7) =63344a(8) =957206a(9) =16368226a(10) =312484331a(11) =6588211317a(12) =152035645781