a(n) = (2*n)! - ((2*n-1)!!)^2.A151816a(n) = (2*n)! - ((2*n-1)!!)^2.Terms a(0) =0a(1) =1a(2) =15a(3) =495a(4) =29295a(5) =2735775a(6) =370945575a(7) =68916822975External referencesoeis: A151816