a(n) = T(2n,n+1), T given by A026736.

Terms

a(0) =1a(1) =4a(2) =15a(3) =57a(4) =221a(5) =872a(6) =3489a(7) =14113a(8) =57575a(9) =236457a(10) =976271a(11) =4047871a(12) =16840879a(13) =70259892a(14) =293790127a(15) =1230783085a(16) =5164196117a(17) =21696512073a(18) =91254256589a(19) =384165925259