  We study the puzzle graphs of hexagonal sliding puzzles of various shapes and
with various numbers of holes. The puzzle graph is a combinatorial model which
captures the solvability and the complexity of sequential mechanical puzzles.
Questions relating to the puzzle graph have been previously studied and
resolved for the 15 Puzzle which is the most famous, and unsolvable, square
sliding puzzle of all time. It is known that for square puzzles such as the 15
Puzzle, solvability depends on a parity property that splits the puzzle graph
into two components. In the case of hexagonal sliding puzzles, we get more
interesting parity properties that depend on the shape of the boards and on the
missing tiles or holes on the board. We show that for large-enough hexagonal,
triangular, or parallelogram-shaped boards with hexagonal tiles, all puzzles
with three or more holes are solvable. For puzzles with two or more holes, we
give a solvability criterion involving both a parity property and the placement
of tiles in tight corners of the board. The puzzle graph is a discrete model
for the configuration space of hard tiles (hexagons or squares) moving on
different tessellation-based domains. Understanding the combinatorics of the
puzzle graph could lead to understanding some aspects of the topology of these
configuration spaces.
