a(n) = A027113(n, 2n-6).

Terms

a(0) =1a(1) =2a(2) =7a(3) =24a(4) =81a(5) =270a(6) =886a(7) =2861a(8) =9109a(9) =28652a(10) =89211a(11) =275420a(12) =844320a(13) =2573161a(14) =7803656a(15) =23569375a(16) =70941900a(17) =212911167a(18) =637427166a(19) =1904416809a(20) =5679745126a(21) =16913962409a(22) =50304623519a(23) =149450737592a(24) =443593524167