OFFSET
1,3
LINKS
Lucas A. Brown, Python program.
Eric Weisstein's World of Mathematics, Convex Polygon.
Eric Weisstein's World of Mathematics, Quadrilateral.
EXAMPLE
a(1) = 0 because the 1 X 1 grid has no quadrilaterals.
a(2) = 1 because the 2 X 2 grid has one quadrilateral.
a(3) = 9 because the 3 X 3 grid has 12 congruence classes of quadrilaterals, out of 70 quadrilaterals total:
+-------+-------+-------+-------+
| . . . | . o . | . . . | . o . |
| o o . | o . . | o . o | o . . |
| o o . | o o . | o . o | o . o |
+-------+-------+-------+-------+
| . . o | o . o | . o . | . o . |
| o . . | . . . | o o . | o . o |
| o . o | o . o | o . . | o . . |
+-------+-------+-------+-------+
| . o o | . . o | . o . | . . o |
| o . . | o . o | o . o | o . . |
| o . . | o . . | . o . | o o . |
+-------+-------+-------+-------+
CROSSREFS
KEYWORD
nonn
AUTHOR
Martin Renner, Apr 03 2012
EXTENSIONS
a(7)-a(33) from Lucas A. Brown, Feb 06 2024
STATUS
approved