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Cauchy Theorem
(redirected from Cauchy's theorem)

   Also found in: Wikipedia 0.01 sec.
Cauchy Theorem 

a theorem concerned with the expansion of an analytic function into a power series. Suppose f(z) is a function that is single-valued and analytic in a region G, Z0 is an arbitrary (finite) point of G, and ρ is the distance from z0 to the boundary of this region. Then there exists a power series in z – z0 that converges to the function in the interior of the circle ǀz – z0ǀ = ρ:

If the boundary of G reduces to the point at infinity, then ρ is infinite. This theorem was established by A. Cauchy (1831), who based it on his representation of an analytic function in the form of the so-called Cauchy integral.



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He continues with Abel's theorem, the gamma function, universal covering spaces, Cauchy's theorem for non-holomorphic functions and harmonic conjugates.
Cauchy's theorem may then be used to derive the relationship between the real and imaginary parts of such a function, known in physics as the Kramers-Kronig relation.
We only need to expand again the integrand of (30) (with some care) and collect the main terms: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Inserting this last approximation into (30) and completing the tails then yields [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (31) The integral can be evaluated by shifting vertically the integration line and making use of Cauchy's Theorem.
 
 
 
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