a(n) = T(2n,n+1), where T is the array in A026268.

Terms

a(0) =1a(1) =6a(2) =38a(3) =229a(4) =1349a(5) =7894a(6) =46122a(7) =269607a(8) =1578035a(9) =9251060a(10) =54322938a(11) =319502765a(12) =1882040458a(13) =11102029850a(14) =65576732025a(15) =387817554915