A sequence related to the even-indexed Catalan numbers.

Terms

a(0) =1a(1) =2a(2) =6a(3) =20a(4) =78a(5) =332a(6) =1516a(7) =7240a(8) =35734a(9) =180620a(10) =929940a(11) =4858328a(12) =25687052a(13) =137177016a(14) =738819672a(15) =4008435984a(16) =21886788582a(17) =120178329740a(18) =663179894788