a(n) = T(2n-1,n-1), T given by A026648.

Terms

a(0) =1a(1) =4a(2) =14a(3) =57a(4) =222a(5) =928a(6) =3764a(7) =15971a(8) =66190a(9) =283464a(10) =1190628a(11) =5131002a(12) =21749532a(13) =94153056a(14) =401757672a(15) =1745117091a(16) =7484144598a(17) =32595240952a(18) =140342565620a(19) =612526487822