a(n) = (2*n+1)! / n!.A000407a(n) = (2*n+1)! / n!.Terms a(0) =1a(1) =6a(2) =60a(3) =840a(4) =15120a(5) =332640a(6) =8648640a(7) =259459200a(8) =8821612800a(9) =335221286400External referencesoeis: A000407