Invariant number of polygons of n-circum-C_5 H_5 systems.

Terms

a(0) =1a(1) =6a(2) =21a(3) =61a(4) =166a(5) =441a(6) =1161a(7) =3046a(8) =7981a(9) =20901a(10) =54726a(11) =143281a(12) =375121a(13) =982086a(14) =2571141a(15) =6731341a(16) =17622886a(17) =46137321a(18) =120789081a(19) =316229926a(20) =827900701a(21) =2167472181a(22) =5674515846a(23) =14856075361a(24) =38893710241a(25) =101825055366a(26) =266581455861a(27) =697919312221