a(n) = A108625(n,2*n).

Terms

a(0) =1a(1) =5a(2) =61a(3) =923a(4) =15421a(5) =272755a(6) =5006275a(7) =94307855a(8) =1811113021a(9) =35301145037a(10) =696227550811