a(n)=f(n,n-1) where f is given in A034261.

Terms

a(0) =1a(1) =5a(2) =25a(3) =119a(4) =546a(5) =2442a(6) =10725a(7) =46475a(8) =199342a(9) =848198a(10) =3585946a(11) =15080870a(12) =63146500a(13) =263432340a(14) =1095517485a(15) =4543460595a(16) =18798494550a(17) =77616288750a(18) =319874637390