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Fixed point theory concerns itself with a very simple, and basic,
mathematical setting. For a functionf that has a setX as bothdomain
and range, a ?xed point off isa pointx ofX for whichf(x)=x. Two
fundamental theorems concerning ?xed points are those of Banach and
of Brouwer. In Banach's theorem, X is a complete metric space with
metricd andf:X?X is required to be a contraction, that is, there
must existL< 1 such thatd(f(x),f(y))?Ld(x,y) for allx,y?X.
Theconclusion is thatf has a ?xed point, in fact exactly one of
them. Brouwer'stheorem requiresX to betheclosed unit ball in a
Euclidean space and f:X?X to be a map, that is, a continuous
function. Again we can conclude that f has a ?xed point. But in
this case the set of?xed points need not be a single point, in fact
every closed nonempty subset of the unit ball is the ?xed point set
for some map. ThemetriconX in Banach'stheorem is used in the
crucialhypothesis about the function, that it is a contraction. The
unit ball in Euclidean space is also metric, and the metric
topology determines the continuity of the function, but the focus
of Brouwer's theorem is on topological characteristics of the unit
ball, in particular that it is a contractible ?nite polyhedron. The
theorems of Banach and Brouwer illustrate the di?erence between the
two principal branches of ?xed point theory: metric ?xed point
theory and topological ?xed point theory.
This selection of papers from the Beijing conference gives a
cross-section of the current trends in the field of fixed point
theory as seen by topologists and analysts. Apart from one survey
article, they are all original research articles, on topics
including equivariant theory, extensions of Nielsen theory,
periodic orbits of discrete and continuous dynamical systems, and
new invariants and techniques in topological approaches to analytic
problems.
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