%I #4 Feb 27 2009 03:00:00
%S 1,1,1,2,5,13,37,112,365,1253,4542,17149,67455,274134,1150125
%N Number of bi-secondary structures of size N.
%C A BiSecondary Structure(BSS) is a RNA graph that can be drawn without crossings using at most two halfplanes. The numbers displayed here were obtained by first generating all secondary structures of a given size and then by carefully counting possible completions on the second halfplane. Keep in mind that the simple shuffling of two secondary structures would yield many isomorphic RNA graphs.
%e Encoding the secondary structure on the first halfplane with '()', on the 2nd with '[]' and unpaired bases with '-'. For n=4, bss(n)=5:
%e .... (.). (..) .(.) ([)]
%e Notice that '[(])' must not be counted, as it is isomorphic to '([)]'.
%Y Cf. A004148.
%K easy,nonn
%O 0,4
%A Yann Ponty (Yann.Ponty(AT)lri.fr), May 25 2006