OFFSET
1,2
COMMENTS
Posets whose Hasse diagram looks the same if it is turned upside down. - N. J. A. Sloane, Mar 22 2013
REFERENCES
R. P. Stanley, Enumerative Combinatorics I, 2nd. ed., p. 247.
PROG
(Sage) for n in range(1, 10): print(len([P for P in Posets(n) if P.is_selfdual()])) # Jori Mäntysalo, Oct 02 2014
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Richard Stanley, Jan 08 2008
EXTENSIONS
a(9) from Jori Mäntysalo, Oct 02 2014
a(10-12) from Rico Zöllner and Konrad Handrich, Oct 22 2024
STATUS
approved