The Eighth Week: Coverings
Michio Kuga · Birkhäuser Boston eBooks · 1993
Let us start with an example. Take two distinct points O’ and O in the plane. Let a and b be real numbers such that 0 < a < b. Let D’ be the set of points P whose distance from O’ is between a and b. Similarly, let D be the region bounded by two circles of radii a and b and center O. See Figure 8.1.