a(n) = A026615(2*n, n-1).

Terms

a(0) =1a(1) =7a(2) =26a(3) =97a(4) =364a(5) =1374a(6) =5214a(7) =19877a(8) =76076a(9) =292162a(10) =1125332a(11) =4345642a(12) =16819256a(13) =65226620a(14) =253403190a(15) =986022765a(16) =3842200140a(17) =14991031770a(18) =58558504620a(19) =228986816190a(20) =896300806440