# Circle of Convergence

## circle of convergence

[′sər·kəl əv kən′vər·jəns]
(mathematics)
The region in which a power series possesses a limit.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Circle of Convergence

The circle of convergence of a power series

(*) a0 + a1(z - z0)2 + a2(z - z0)2 + …

is a circle ǀz – z0ǀ = R in the plane of the complex variable z such that the series (*) con verges if ǀz – aǀ < R and diverges if ǀz – aǀ > R (the series may both converge and diverge at points on the circle ǀz – z0ǀ = R). Every power series converges in the entire plane (for any z), has a circle of convergence of finite radius R, or converges only when z = Z0. The series (*) converges to an analytic function in the circle of convergence. The number R is called the radius of convergence of the series (*) and determined from the Cauchy-Hadamard formula If z0 = x0 is a real number, then the part of the real axis Ox lying in the circle of convergence is called the interval of convergence.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
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