Lakes of Wada

Three disjoint sets that share a common boundary

In mathematics, the lakes of Wada (和田の湖) are three disjoint connected open sets of the plane or open unit square with the counterintuitive property that they all have the same boundary. In other words, for any point selected on the boundary of one of the lakes, the other two lakes' boundaries also contain that point. More than two sets with the same boundary are said to have the Wada property; examples include Wada basins in dynamical systems. This property is rare in real world systems. The lakes of Wada were introduced by , who credited the discovery to Takeo Wada. His construction is similar to the construction by of an indecomposable continuum, and in fact it is possible for the common boundary of the three sets to be an indecomposable continuum.

Wada basins are certain special basins of attraction studied in the mathematics of non linear systems. A basin having the property that every neighborhood of every point on the boundary of that basin intersects at least three basins is called a Wada basin, or said to have the Wada property. Unlike the Lakes of Wada, Wada basins are often disconnected. An example of Wada basins is given by the Newton fractal describing the basins of attraction of the Newton–Raphson method for finding the roots of a cubic polynomial with distinct roots, such as z3 − 1; see the picture.

In chaos theory, Wada basins arise very frequently. Usually, the Wada property can be seen in the basin of attraction of dissipative dynamical systems. But the exit basins of Hamiltonian systems can also show the Wada property. In the context of the chaotic scattering of systems with multiple exits, basins of exits show the Wada property. M. A. F. Sanjuán et al. has shown that in the Hénon–Heiles system the exit basins have this Wada property.