a(n) = T(2n+1,n+2), T given by A026747.

Terms

a(0) =1a(1) =6a(2) =29a(3) =132a(4) =589a(5) =2613a(6) =11592a(7) =51558a(8) =230181a(9) =1032060a(10) =4648150a(11) =21027765a(12) =95542878a(13) =435939525a(14) =1997076805a(15) =9183661080a(16) =42383777344a(17) =196271453865a(18) =911804206063