a(n) = T(n,n+2) where T is the array defined in A025564.

Terms

a(0) =1a(1) =5a(2) =19a(3) =65a(4) =211a(5) =665a(6) =2058a(7) =6294a(8) =19095a(9) =57607a(10) =173096a(11) =518596a(12) =1550367a(13) =4627455a(14) =13795176a(15) =41088456a(16) =122297643a(17) =363828663a(18) =1081966875a(19) =3216725841a(20) =9561635853a(21) =28418162003a(22) =84455354206a(23) =250982289650a(24) =745860104145