a(n) = T(2n,n+1), where T is the array in A026300.

Terms

a(0) =0a(1) =2a(2) =12a(3) =69a(4) =392a(5) =2235a(6) =12804a(7) =73710a(8) =426192a(9) =2473704a(10) =14405800a(11) =84137130a(12) =492652824a(13) =2891110235a(14) =16999928820a(15) =100136858625a(16) =590778928800