a(n) = n!! mod !n.

Terms

a(0) =0a(1) =1a(2) =8a(3) =15a(4) =48a(5) =105a(6) =384a(7) =945a(8) =3840a(9) =10395a(10) =46080a(11) =135135a(12) =645120a(13) =2027025a(14) =10321920a(15) =34459425a(16) =185794560a(17) =654729075a(18) =3715891200a(19) =13749310575a(20) =81749606400a(21) =316234143225