# Rouché's theorem

Rouché's theorem For the theorem in linear algebra, see Rouché–Capelli theorem. Mathematical analysis → Complex analysis Complex analysis Complex numbers Real numberImaginary numberComplex planeComplex conjugateUnit complex number Complex functions Complex-valued functionAnalytic functionHolomorphic functionCauchy–Riemann equationsFormal power series Basic Theory Zeros and polesCauchy's integral theoremLocal primitiveCauchy's integral formulaWinding numberLaurent seriesIsolated singularityResidue theoremConformal mapSchwarz lemmaHarmonic functionLaplace's equation Geometric function theory People Augustin-Louis CauchyLeonhard EulerCarl Friedrich GaussJacques HadamardKiyoshi OkaBernhard RiemannKarl Weierstrass Mathematics portal vte Rouché's theorem, named after Eugène Rouché, states that for any two complex-valued functions f and g holomorphic inside some region {displaystyle K} with closed contour {displaystyle partial K} , if |g(z)| < |f(z)| on {displaystyle partial K} , then f and f + g have the same number of zeros inside {displaystyle K} , where each zero is counted as many times as its multiplicity. This theorem assumes that the contour {displaystyle partial K} is simple, that is, without self-intersections. Rouché's theorem is an easy consequence of a stronger symmetric Rouché's theorem described below. Contents 1 Usage 2 Geometric explanation 3 Applications 4 Symmetric version 5 Proof of the symmetric form of Rouché's theorem 6 See also 7 Notes 8 References Usage The theorem is usually used to simplify the problem of locating zeros, as follows. Given an analytic function, we write it as the sum of two parts, one of which is simpler and grows faster than (thus dominates) the other part. We can then locate the zeros by looking at only the dominating part. For example, the polynomial {displaystyle z^{5}+3z^{3}+7} has exactly 5 zeros in the disk {displaystyle |z|<2} since {displaystyle |3z^{3}+7|leq 31<32=|z^{5}|} for every {displaystyle |z|=2} , and {displaystyle z^{5}} , the dominating part, has five zeros in the disk. Geometric explanation Since the distance between the curves is small, h(z) does exactly one turn around just as f(z) does. It is possible to provide an informal explanation of Rouché's theorem. Let C be a closed, simple curve (i.e., not self-intersecting). Let h(z) = f(z) + g(z). If f and g are both holomorphic on the interior of C, then h must also be holomorphic on the interior of C. Then, with the conditions imposed above, the Rouche's theorem in its original (and not symmetric) form says that If |f(z)| > |h(z) − f(z)|, for every z in C, then f and h have the same number of zeros in the interior of C.

Notice that the condition |f(z)| > |h(z) − f(z)| means that for any z, the distance from f(z) to the origin is larger than the length of h(z) − f(z), which in the following picture means that for each point on the blue curve, the segment joining it to the origin is larger than the green segment associated with it. Informally we can say that the blue curve f(z) is always closer to the red curve h(z) than it is to the origin.

The previous paragraph shows that h(z) must wind around the origin exactly as many times as f(z). The index of both curves around zero is therefore the same, so by the argument principle, f(z) and h(z) must have the same number of zeros inside C.

One popular, informal way to summarize this argument is as follows: If a person were to walk a dog on a leash around and around a tree, such that the distance between the person and the tree is always greater than the length of the leash, then the person and the dog go around the tree the same number of times.

Applications See also: Properties of polynomial roots § Bounds on (complex) polynomial roots Consider the polynomial {displaystyle z^{2}+2az+b^{2}} (where {displaystyle a>b>0} ). By the quadratic formula it has two zeros at {displaystyle -apm {sqrt {a^{2}-b^{2}}}} . Rouché's theorem can be used to obtain more precise positions of them. Since {displaystyle |z^{2}+b^{2}|leq 2b^{2}<2a|z|} for every {displaystyle |z|=b} , Rouché's theorem says that the polynomial has exactly one zero inside the disk {displaystyle |z|**0} so large that: {displaystyle |a_{0}+a_{1}z+cdots +a_{n-1}z^{n-1}|leq sum _{j=0}^{n-1}|a_{j}|R^{j}<|a_{n}|R^{n}=|a_{n}z^{n}|{text{ for }}|z|=R.} Since {displaystyle a_{n}z^{n}} has {displaystyle n} zeros inside the disk {displaystyle |z| 0} ), it follows from Rouché's theorem that {displaystyle p} also has the same number of zeros inside the disk.**

One advantage of this proof over the others is that it shows not only that a polynomial must have a zero but the number of its zeros is equal to its degree (counting, as usual, multiplicity).

Another use of Rouché's theorem is to prove the open mapping theorem for analytic functions. We refer to the article for the proof.

Symmetric version A stronger version of Rouché's theorem was published by Theodor Estermann in 1962.[1] It states: let {displaystyle Ksubset G} be a bounded region with continuous boundary {displaystyle partial K} . Two holomorphic functions {displaystyle f,,gin {mathcal {H}}(G)} have the same number of roots (counting multiplicity) in {displaystyle K} , if the strict inequality {displaystyle |f(z)-g(z)|<|f(z)|+|g(z)|qquad left(zin partial Kright)} holds on the boundary {displaystyle partial K.} The original version of Rouché's theorem then follows from this symmetric version applied to the functions {displaystyle f+g,f} together with the trivial inequality {displaystyle |f(z)+g(z)|geq 0} (in fact this inequality is strict since {displaystyle f(z)+g(z)=0} for some {displaystyle zin partial K} would imply {displaystyle |g(z)|=|f(z)|} ). The statement can be understood intuitively as follows. By considering {displaystyle -g} in place of {displaystyle g} , the condition can be rewritten as {displaystyle |f(z)+g(z)|<|f(z)|+|g(z)|} for {displaystyle zin partial K} . Since {displaystyle |f(z)+g(z)|leq |f(z)|+|g(z)|} always holds by the triangle inequality, this is equivalent to saying that {displaystyle |f(z)+g(z)|neq |f(z)|+|g(z)|} on {displaystyle partial K} , which in turn means that for {displaystyle zin partial K} the functions {displaystyle f(z)} and {displaystyle g(z)} are non-vanishing and {displaystyle arg {f(z)}neq arg {g(z)}} . Intuitively, if the values of {displaystyle f} and {displaystyle g} never pass through the origin and never point in the same direction as {displaystyle z} circles along {displaystyle partial K} , then {displaystyle f(z)} and {displaystyle g(z)} must wind around the origin the same number of times. Proof of the symmetric form of Rouché's theorem Let {displaystyle Ccolon [0,1]to mathbb {C} } be a simple closed curve whose image is the boundary {displaystyle partial K} . The hypothesis implies that f has no roots on {displaystyle partial K} , hence by the argument principle, the number Nf(K) of zeros of f in K is {displaystyle {frac {1}{2pi i}}oint _{C}{frac {f'(z)}{f(z)}},dz={frac {1}{2pi i}}oint _{fcirc C}{frac {dz}{z}}=mathrm {Ind} _{fcirc C}(0),} i.e., the winding number of the closed curve {displaystyle fcirc C} around the origin; similarly for g. The hypothesis ensures that g(z) is not a negative real multiple of f(z) for any z = C(x), thus 0 does not lie on the line segment joining f(C(x)) to g(C(x)), and {displaystyle H_{t}(x)=(1-t)f(C(x))+tg(C(x))} is a homotopy between the curves {displaystyle fcirc C} and {displaystyle gcirc C} avoiding the origin. The winding number is homotopy-invariant: the function {displaystyle I(t)=mathrm {Ind} _{H_{t}}(0)={frac {1}{2pi i}}oint _{H_{t}}{frac {dz}{z}}} is continuous and integer-valued, hence constant. This shows {displaystyle N_{f}(K)=mathrm {Ind} _{fcirc C}(0)=mathrm {Ind} _{gcirc C}(0)=N_{g}(K).} See also Fundamental theorem of algebra, for its shortest demonstration yet, while using Rouché's theorem Hurwitz's theorem (complex analysis) Rational root theorem Properties of polynomial roots Riemann mapping theorem Sturm's theorem Notes This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please help to improve this article by introducing more precise citations. (May 2015) (Learn how and when to remove this template message) ^ Estermann, T. (1962). Complex Numbers and Functions. Athlone Press, Univ. of London. p. 156. References Beardon, Alan (1979). Complex Analysis: The Argument Principle in Analysis and Topology. John Wiley and Sons. p. 131. ISBN 0-471-99672-6. Conway, John B. (1978). Functions of One Complex Variable I. Springer-Verlag New York. ISBN 978-0-387-90328-6. Titchmarsh, E. C. (1939). The Theory of Functions (2nd ed.). Oxford University Press. pp. 117–119, 198–203. ISBN 0-19-853349-7. Rouché É., Mémoire sur la série de Lagrange, Journal de l'École Polytechnique, tome 22, 1862, p. 193-224. Theorem appears at p. 217. See Gallica archives. Categories: Theorems in complex analysis

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