From [Wikipedia](http://en.wikipedia.org/wiki/Y-%CE%94_transform): _Every two-terminal network represented by a planar graph can be reduced to a single equivalent resistor by a sequence of series, parallel, $Y$-$\Delta$, and $\Delta$-$Y$ transformations. However, there are non-planar networks that cannot be simplified using these transformations, such as a regular square grid wrapped around a torus, or any member of the Petersen family._