 The distance from the center to either of those points is 2, so the radius of convergence is 2.If the power series is expanded around the point "a" and the radius of convergence is , then the set of all points such that is a circle called the "boundary" of the disk of convergence.A power series may diverge at every point on the boundary, or diverge on some points and converge at other points, or converge at all the points on the boundary.