Number of possible column states for self-avoiding polygons in a slit of width n.

Terms

a(0) =1a(1) =3a(2) =8a(3) =20a(4) =50a(5) =126a(6) =322a(7) =834a(8) =2187a(9) =5797a(10) =15510a(11) =41834a(12) =113633a(13) =310571a(14) =853466a(15) =2356778a(16) =6536381a(17) =18199283a(18) =50852018a(19) =142547558a(20) =400763222a(21) =1129760414a(22) =3192727796a(23) =9043402500a(24) =25669818475a(25) =73007772801a(26) =208023278208a(27) =593742784828