a(n) = (3*n)!/(2^n*n!).A166334a(n) = (3*n)!/(2^n*n!).Terms a(0) =1a(1) =3a(2) =90a(3) =7560a(4) =1247400a(5) =340540200a(6) =138940401600External referencesoeis: A166334