a(n) = (2*n+1)!! / ((floor((n-1)/3)*2+1))!!

Terms

a(0) =1a(1) =3a(2) =15a(3) =105a(4) =315a(5) =3465a(6) =45045a(7) =135135a(8) =2297295a(9) =43648605a(10) =130945815a(11) =3011753745a(12) =75293843625a(13) =225881530875