T(2n,n+3), T given by A026747.

Terms

a(0) =1a(1) =9a(2) =56a(3) =301a(4) =1504a(5) =7217a(6) =33821a(7) =156274a(8) =716116a(9) =3266558a(10) =14868789a(11) =67649170a(12) =307997855a(13) =1404343584a(14) =6416130302a(15) =29383457373a(16) =134915391509a(17) =621163581667