a(n) = T(2n,n+1), where T is the array in A026148.

Terms

a(0) =2a(1) =10a(2) =60a(3) =343a(4) =1967a(5) =11308a(6) =65259a(7) =378010a(8) =2197063a(9) =12808546a(10) =74873071a(11) =438719043a(12) =2576121325a(13) =15155321640a(14) =89309379504a(15) =527093971974