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Consider a complex power series (1)

and let If , then the series (1) is convergent only at the point ; if , then the series (1) is absolutely convergent in the disc where (2)

and divergent outside the disc, where ; if , the series (1) is absolutely convergent for all . The content of the Cauchy–Hadamard theorem is thus expressed by the Cauchy–Hadamard formula (2), which should be understood in this context in a broad sense, including and . In other words, the Cauchy–Hadamard theorem states that the interior of the set of points at which the series (1) is (absolutely) convergent is the disc of radius (2). In the case of a real power series (1), formula (2) defines the "radius" of the interval of convergence: . Essentially, the Cauchy–Hadamard theorem was stated by A.L. Cauchy in his lectures  in 1821; it was J. Hadamard  who made the formulation and the proof fully explicit.

For power series (3)

in complex variables , , one has the following generalization of the Cauchy–Hadamard formula: (4) which is valid for the associated radii of convergence of the series (3) (see Disc of convergence). Writing (4) in the form , one obtains an equation defining the boundary of a certain logarithmically convex Reinhardt domain with centre , which is the interior of the set of points at which the series (3) is absolutely convergent ( ).

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