a(n) = (4*n)! / ((2*n)!*n!^2).A000897a(n) = (4*n)! / ((2*n)!*n!^2).Terms a(0) =1a(1) =12a(2) =420a(3) =18480a(4) =900900a(5) =46558512a(6) =2498640144a(7) =137680171200External referencesoeis: A000897