Michael selection theorem

Template:Short description In functional analysis, a branch of mathematics, Michael selection theorem is a selection theorem named after Ernest Michael. In its most popular form, it states the following:

Template:Math theorem Conversely, if any lower semicontinuous multimap from topological space X to a Banach space, with nonempty convex closed values, admits a continuous selection, then X is paracompact. This provides another characterization for paracompactness.

Examples

File:Kakutani.svg

A function that satisfies all requirements

The function:

$F(x)=[1-x/2,1-x/4]$

, shown by the grey area in the figure at the right, is a set-valued function from the real interval [0,1] to itself. It satisfies all Michael's conditions, and indeed it has a continuous selection, for example:

$f(x)=1-x/2$

or

$f(x)=1-3x/8$

.

A function that does not satisfy lower hemicontinuity

The function

$F(x)=\{\begin{array}{ll}3/4& 0\le x<0.5\\ [0,1]& x=0.5\\ 1/4& 0.5<x\le 1\end{array}$

is a set-valued function from the real interval [0,1] to itself. It has nonempty convex closed values. However, it is not lower hemicontinuous at 0.5. Indeed, Michael's theorem does not apply and the function does not have a continuous selection: any selection at 0.5 is necessarily discontinuous.

Applications

Michael selection theorem can be applied to show that the differential inclusion

$\frac{dx}{dt}(t)\in F(t,x(t)),x(t_0)=x_0$

has a C¹ solution when F is lower semi-continuous and F(t, x) is a nonempty closed and convex set for all (t, x). When F is single valued, this is the classic Peano existence theorem.

Generalizations

A theorem due to Deutsch and Kenderov generalizes Michel selection theorem to an equivalence relating approximate selections to almost lower hemicontinuity, where $F$ is said to be almost lower hemicontinuous if at each $x\in X$, all neighborhoods $V$ of $0$ there exists a neighborhood $U$ of $x$ such that ${\cap }_{u\in U}\{F(u)+V\}\ne \mathrm{\varnothing }.$

Precisely, Deutsch–Kenderov theorem states that if $X$ is paracompact, $Y$ a normed vector space and $F(x)$ is nonempty convex for each $x\in X$, then $F$ is almost lower hemicontinuous if and only if $F$ has continuous approximate selections, that is, for each neighborhood $V$ of $0$ in $Y$ there is a continuous function $f:X\mapsto Y$ such that for each $x\in X$, $f(x)\in F(X)+V$.

In a note Xu proved that Deutsch–Kenderov theorem is also valid if $Y$ is a locally convex topological vector space.

See also

• Zero-dimensional Michael selection theorem
• Selection theorem
• Maximum theorem