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A203935

Number of lattice 2n-gons with edges on distinct lines.

a(n) is the number of self-avoiding polygons on the nXn square lattice having 2n edges (arbitrary perimeter) with an edge on each of the n distinct horizontal and n distinct vertical lines in the lattice.

a(4)=26: 4 symmetries of the Z pentomino; 4 rotations of the step (123 bargraph) hexomino; 8 symmetries of each of the 132 and 213 bargraph hexominoes; and 2 rotations of the heptomino that consists of a 3x3 square with opposite corner cells removed.