OFFSET
1,5
COMMENTS
If you checkerboard color the vertices of the square lattice, a(n) is the number of orthogonal polygons of area n, which alternates between white and black vertices.
LINKS
Giovanni Resta, Illustration of a(10) = 55.
EXAMPLE
For n=5 the a(5)=2 pentominoes are 'I' and 'X'.
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Thomas Dybdahl Ahle, May 28 2013
EXTENSIONS
a(10) from Giovanni Resta, Jun 06 2013
a(11)-a(12) from Thomas Dybdahl Ahle, Nov 13 2019
a(13)-a(15) from Aurélien Ooms, Nov 14-19 2019
STATUS
approved