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Rouché's Theorem


Given two functions f and g analytic in A with gamma a simple loop homotopic to a point in A, if |g(z)|<|f(z)| for all z on gamma, then f and f+g have the same number of roots inside gamma.

A stronger version has been proved by Estermann (1962). The strong version also has a converse, as shown by Challener and Rubel (1982).


See also

Argument Principle

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References

Challener, D. and Rubel, L. "A Converse to Rouché's Theorem." Amer. Math. Monthly 89, 302-305, 1982.Estermann, T. Complex Numbers and Functions. London: Oxford University Press, p. 156, 1962.Knopp, K. Theory of Functions Parts I and II, Two Volumes Bound as One, Part II. New York: Dover, p. 111, 1996.Krantz, S. G. "Rouché's Theorem." §5.3.1 in Handbook of Complex Variables. Boston, MA: Birkhäuser, p. 74, 1999.Szegö, G. Orthogonal Polynomials, 4th ed. Providence, RI: Amer. Math. Soc., p. 22, 1975.

Referenced on Wolfram|Alpha

Rouché's Theorem

Cite this as:

Weisstein, Eric W. "Rouché's Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/RouchesTheorem.html

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