Hurwitz's theorem

In mathematics, Hurwitz's theorem refers to at least four different results, each named after Adolf Hurwitz.

Hurwitz's theorem in complex analysis

In complex analysis, Hurwitz's theorem roughly states that, under certain conditions, if a sequence of holomorphic functions converges uniformly to a holomorphic function on compact sets, then after a while those functions and the limit function have the same number of zeros in any open disk.

More precisely, let $G$ be an open set in the complex plane, and consider a sequence of holomorphic functions $(f_{n})$ which converges uniformly on compact subsets of $G$ to a holomorphic function $f.$ Let $D(z_{0},r)$ be an open disk of center $z_{0}$ and radius $r$ which is contained in $G$ together with its boundary. Assume that $f(z)$ is non-zero on the disk boundary. Then, there exists a natural number $N$ such that for all $n$ greater than $N$ the functions $f_{n}$ and $f$ have the same number of zeros in $D(z_{0},r).$

The requirement that $f$ be nonzero on the disk boundary is necessary. For example, consider the disk of center zero and radius 1, and the sequence

$f_{n}(z)=z-1+{\frac {1}{n}}$

for all $z.$ It converges uniformly to $f(z)=z-1$ which has no zeros inside of this disk, but each $f_{n}(z)$ has exactly one zero in the disk, which is $1-1/n.$

This result holds more generally for any bounded convex sets but it is most useful to state for disks.

An immediate consequence of this theorem is the following corollary. If $G$ is an open set and a sequence of holomorphic functions $(f_{n})$ converges uniformly on compact subsets of $G$ to a holomorphic function $f,$ and furthermore if $f_{n}$ is not zero at any point in $G$, then $f$ is either identically zero or also is never zero.

References

• John B. Conway. Functions of One Complex Variable I. Springer-Verlag, New York, New York, 1978.

Hurwitz's theorem at PlanetMath.

Hurwitz's theorem in algebraic geometry

Main article: Riemann-Hurwitz formula

In algebraic geometry, the result referred to as Hurwitz's theorem is an index theorem which relates the degree of a branched cover of algebraic curves, the genera of these curves and the behaviour of f at the branch points.

More explicitly, let $f:X\rightarrow Y$ be a finite morphism of curves over an algebraically closed field, and suppose that f is tamely ramified.

Let R be the ramification divisor

$R=\sum _{P\in X}(e_{P}-1)P,$

where $e_{P}$ denotes the ramification index of f at P. Let n = deg f, and let g(X), g(Y) denote the genus of X, Y respectively.

Then Hurwitz's theorem states that

2g(X) − 2 = n(2g(Y) − 2) + deg R.

References

• R. Hartshorne, Algebraic Geometry, Springer, New York 1977

Hurwitz's theorem for composition algebras

In this context, Hurwitz's theorem states that the only composition algebras over ${\mathbb {R}}$ are ${\mathbb {R}}$, $\mathbb {C}$, $\mathbb {H}$ and $\mathbb {O}$, that is the real numbers, the complex numbers, the quaternions and the octonions.

References

• John H. Conway, Derek A. Smith On Quaternions and Octonions. A.K. Peters, 2003.

Hurwitz's theorem in number theory

In number theory, the Hurwitz's theorem states that for every irrational number $\xi$ there are infinitely many rationals m/n such that

$\left|\xi -{\frac {m}{n}}\right|<{\frac {1}{{\sqrt {5}}n^{2}}}.$

Here the constant ${\sqrt {5}}$ is the best possible; if we substitute ${\sqrt {5}}$ by any number A > 5^1/2 we can find only finite many rational approximations such that the formula above holds.

References

• G. H. Hardy, E. M. Wright An introduction to the Theory of Numbers, fifth edition, Oxford science publications, 2003.