Double factorials n!!: a(n) = n*a(n-2) for n > 1, a(0) = a(1) = 1.

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =8a(5) =15a(6) =48a(7) =105a(8) =384a(9) =945a(10) =3840a(11) =10395a(12) =46080a(13) =135135a(14) =645120a(15) =2027025a(16) =10321920a(17) =34459425a(18) =185794560a(19) =654729075a(20) =3715891200a(21) =13749310575a(22) =81749606400a(23) =316234143225