Product_{k=1..n} floor(2*n/k - 1).

Terms

a(0) =1a(1) =1a(2) =3a(3) =10a(4) =21a(5) =72a(6) =330a(7) =468a(8) =2520a(9) =8160a(10) =20520a(11) =60480a(12) =318780a(13) =504000a(14) =2426112a(15) =10523520a(16) =21092400a(17) =53222400a(18) =452390400a(19) =506373120a(20) =3226728960a(21) =11604902400a(22) =21299241600a(23) =76640256000a(24) =431500608000a(25) =844958822400