a(n) = T(n,2n-2), T given by A027023.

Terms

a(0) =1a(1) =5a(2) =13a(3) =33a(4) =85a(5) =221a(6) =581a(7) =1545a(8) =4149a(9) =11237a(10) =30657a(11) =84169a(12) =232361a(13) =644573a(14) =1795717a(15) =5021801a(16) =14091829a(17) =39665893a(18) =111965785a(19) =316857945a(20) =898797441a(21) =2555025821a(22) =7277679961a(23) =20767821489a(24) =59365259065a(25) =169967668645a(26) =487356812589