T(2n,n+1), T given by A026780.

Terms

a(0) =1a(1) =5a(2) =23a(3) =106a(4) =496a(5) =2361a(6) =11425a(7) =56128a(8) =279530a(9) =1409242a(10) =7182570a(11) =36965292a(12) =191896092a(13) =1003889999a(14) =5288020783a(15) =28026601758a(16) =149362212188a(17) =799947847992