Even Catalan numbers.

Terms

a(0) =2a(1) =14a(2) =42a(3) =132a(4) =1430a(5) =4862a(6) =16796a(7) =58786a(8) =208012a(9) =742900a(10) =2674440a(11) =35357670a(12) =129644790a(13) =477638700a(14) =1767263190a(15) =6564120420a(16) =24466267020a(17) =91482563640a(18) =343059613650