a(n) = T(2n,n+3), T given by A026725.

Terms

a(0) =1a(1) =8a(2) =46a(3) =233a(4) =1108a(5) =5083a(6) =22805a(7) =100827a(8) =441311a(9) =1917751a(10) =8289965a(11) =35694218a(12) =153225617a(13) =656213596a(14) =2805143526a(15) =11973556060a(16) =51047361676a(17) =217420991444a(18) =925300665762