| A geometrical construction almost a hunderd years old, due to Yoneyama
and attributed to Wada, begins like this. |
|
| In a red ocean an island has a
green lake and a blue lake. |
| A canal is dug from the
ocean so every piece of land on the island
is within 1 mile of ocean water. |
| Another canal is dug from the
blue lake so every piece of land on the
island is within 1/2 mile of blue lake water. |
| A third canal is dug from the
green lake so every piece of land on the
island is within 1/4 mile of green lake water. |
| A fourth canal is dug from the
ocean
so every piece of land on the island
is within 1/8 mile of ocean water. |
| and so on. |
| After all the canals are dug, every point of the island is a boundary point of
both lakes and the ocean. |
| This is the first example of the Wada property. |
| In this example, note that every boundary point can be reached by traveling
through canals from each body of water. We say the boundary points are
accessible from the bodies of water. |
| Now some authors relax the accessibility condition and require only that
each boundary point is a boundary point of all basins. |
