a(n) = (2*n)!/(n+1)!.A001761a(n) = (2*n)!/(n+1)!.Terms a(0) =1a(1) =1a(2) =4a(3) =30a(4) =336a(5) =5040a(6) =95040a(7) =2162160a(8) =57657600a(9) =1764322560a(10) =60949324800External referencesoeis: A001761