The French White Paper on defence and national security

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• cours - matière potentielle : track
• cours - matière potentielle : for high officials

PRÉSIDENCE DE LA RÉPUBLIQUE ______ The French White Paper on defence and national security

• strategic functions
• combat aircraft
• foreign operations
• armed forces
• national territory
• capability
• national security
• defence

Sujets

• NATO
• National Intelligence Council
• National Security Council
• Lisbon
• United Nations

Informations

• Publié par: kevoh
• Nombre de lectures: 19
• Langue: English

Extrait

METRIC TOPOLOGY: A FIRST COURSE
(FOR MATH 4450, SPRING 2011)
PAUL BANKSTON, MARQUETTE UNIVERSITY
Course Contents
The course is divided up into thirty roughly-hour-long lectures.
1. A Historical Introduction (p.2)
2. Sets and Set Operations (p.5)
3. Functions (p.8)
4. Equivalence Relations (p.11)
5. Countable and Uncountable Sets (p.13)
6. The Real Line (p.16)
7. Metric Spaces: Some Examples (p.20)
8. Open Sets and Closed Sets (p.24)
9. Accumulation Points and Convergent Sequences (p.28)
10. Interior, Closure, and Frontier (p.31)
11. Continuous Functions and Homeomorphisms (p.34)
12. Topologically Equivalent Metrics (p.38)
13. Subspaces and Product Spaces (p.40)
14. Complete Metric Spaces I (p.43)
15. Complete Metric Spaces II (p.46)
16. Quotient Spaces I (p.50)
17. Quotient Spaces II (p.54)
18. Separation Properties (p.56)
19. Introduction to Connectedness (p.59)
20. Connected Subsets of Euclidean Space (p.62)
21. Path Connectedness (p.65)
22. Local Connectedness (p.69)
23. Introduction to Compactness (p.71)
24. More on Compactness (p.74)
25. Other Forms of Compactness (p.77)
26. Countability Properties of Metric Spaces (p.80)
27. Introduction to Continua (p.84)
28. Irreducibility (p.88)
29. Cut Points (p.91)
30. Proper Subcontinua (p.94)
Further Reading (p.97)
12
Lecture 1: A Historical Introduction
Abstract. We give a brief historical introduction to topology, and focus on
the development of Euler’s famous theorem concerning spherical polyhedra:
take the number of vertices, subtract the number of edges, add the number of
faces, and the result is invariably 2.
Topology, the area of mathematics sometimes whimsically referred to as “rubber
sheet geometry,” is concerned with the study of properties of a geometric object
that remain unaffected when the object is twisted, stretched, or folded (but not
torn or punctured).
Inhighschoolgeometrytwoobjectsaregeometricallysimilarifonecanbetrans-
formed into the other via a geometric transformation; i.e., a composition of linear
translations, rotations, and dilations. Note that any such composition is invert-
ible, and that its inverse is also a composition of linear translations, rotations, and
dilations. Geometric transformations preserve such intuitive properties as angle
measurement and straightness, but not size. For example any two circles are geo-
metrically similar, as are any two isosceles right triangles or any two line segments.
No triangle is geometrically similar to any polygon with more than three sides,
however.
Intopologywhatconstitutes“similarity”ismuchmoregeneral, inthesensethat
what constitutes a “transformation” is much broader. Two objects are topologi-
cally similar if one can be continuously deformed into the other via a topological
transformation; i.e., a continuous one-to-one correspondence whose inverse is also
continuous. Anothernamefor topological transformation is homeomorphism, andit
isagoalofthiscoursetomakethisideamathematicallysoundandunderstandable.
Only a circle is geometrically similar to a given circle, but there are lots of other
geometric objects that are topologically similar (i.e., homeomorphic) to it. Imagine
your circle to be an elastic band. You can crimp it at various points to form a
triangle or square; stretch it into a large oval; even cut it, tie it into a knot, and
rejoin the ends. All these objects are homeomorphic to a circle, and all go under
the heading of simple closed curve.
So what isn’t homeomorphic to a circle? By the end of this course you will be
able to prove mathematically that the following are not simple closed curves: line
segments, figure-eight curves, spheres, and disks.
Thewordtopology derivesfromGreek,andliterallymeans“analysisofposition.”
The corresponding Latin term is analysis situ¯s, and was the more popular name
for our subject early in the 20th century. How the Greek term ultimately gained
prominence—from the 1920s on—is a subject for the historians of mathematics.
As an autonomous mathematical subject, topology did not really get off the
ground until the late 19th century. By that time mathematics was entering its
“modern”phase,characterizedbybeingfoundeduponsettheory. Toamathematician—
especially one interested in foundations—no mathematical concept is clear until it
can be framed in terms of sets. The term set itself, however, along with the notion
of what it means for something to be a member of a given set, is an undefined—
supposedly intuitively clear—concept. That said, if you look up set in the Oxford
English Dictionary, you’ll find several column inches devoted to its various mean-
ings. In the context of this course, we offer the synonyms class, family, collection,
ensemble and hope for the best. We’ll have more to say about this later.3
Historically, the first known topological result was proved almost four hundred
years ago by none other than Ren´e Descartes of Je pense donc je suis fame. He
was studying the classic polyhedra ofantiquity—e.g., tetrahedra, cubes, octahedra,
etc.—and discovered that the number F of polygonal faces, plus the number V of
vertices exceeds the number E of edges by 2. (Try it out with a cube: there are
six square faces and eight vertices, so F +V =14. E =12; voila`! Now try it with,
say, an octahedron.)
Although we now recognize the result to be topological (for reasons to be given
below), Descarte’s original proof cannot be said to have been one of a topological
flavor. The same assessment goes for the later (18th century) rediscovery of the
result by Leonhard Euler, who formulated it as V −E +F = 2: both arguments
made use of angle measure and relied on the straightness of the edges and the
flatness of the faces.
The first truly topological version of this wonderful result is really due to Henri
Poincar´e in 1895. He realized its essential rubbery nature in the following way:
Imagine that, on the surface of a more-or-less spherical balloon, you’ve marked off
vertices, edges, and faces, much as if you were designing a soccer ball. The simplest
soccer ball would have three vertices, each vertex would be joined to each other
vertex, and there would be two curvy triangular faces (so V = E = 3, F = 2,
and V −E +F = 2). The classic soccer ball is a bit more fancy: twelve (black)
pentagonal faces and twenty (white) hexagonal faces (so F = 32). One easily
checks that V = 12×5 = 60 and that E = (12×5)+30 = 90. Thus, here too,
V −E +F = 60−90+32 = 2. The essential observation is this: Every edge has
two incident vertices (its end points) and forms part of the boundary of two faces.
Suppose there’s a polygonal face with n > 3 edges. We introduce a new vertex in
the interior of that face, as well as edges connecting the new vertex to each vertex
of that face. This means we’ve incremented V by 1, E by n, and F by n− 1;
hence, by this process of triangulation, we have not changed the alternating sum
V −E +F at all. The upshot of this discussion is that we may assume, without
loss of generality, that our original curvy polyhedron has triangular faces only.
Now suppose we remove one vertex and alln of its incident edges from this poly-
hedron with only triangular faces (e.g., tetrahedra, octahedra, icosahedra, but not
cubes and standard soccer balls). Then what we have left is an n-sided polygonal
“hole” in the balloon. Note that we have reduced V by 1 and both E and F by
the same number n; hence V −E +F has been reduced by 1. It suffices to show
that this new alternating sum is 1. Now, since we have a rubbery surface with
a polygonal hole in it, we can “squash” it down onto the plane. This gives us a
polygon which has been subdivided into triangles, where each side of the polygon
is a side of one of the triangles. Any one of these triangles has either 0,1,2, or 3
edges that meet the complement of the polygon. If the number is 0, we call the
triangle interior; otherwise it’s called exterior. IftriangleT is exterior with 3edges
bounding the complement, then it’s the only triangle in the polygon, and we have
V −E+F =3−3+1=1. IfT is exterior with 2 edges bounding the complement,
then its removal decrementsV by 1, E by 2, andF by 1; henceV −E+F is decre-
mented by 1−2+1 = 0. If T is exterior with 1 edge bounding the complement,
then its removal decrements V by 0, E by 1, and F by 1. Hence V −E +F is
decremented by 0−1+1 = 0 in this case too. The process of removing exterior
trianglesmustterminateeventually; hencewhateverwestartedoutwithasthesum4
V −E+F, we never altered it until we arrived at a single triangle, where the sum
is 1. (The reader may recognize this method of argument as an informal version of
the principle of mathematical induction.)
ThealternatingsumV−E+F forsphericalpolyhedraiswellknownastheEuler
2 2characteristic χ(S ) of the sphereS (or of anything homeomorphic to the sphere).
Poincar´e defined this number for a wide variety of geometric objects, proving it to
be a homeomorphism invariant.
Exercises 1. (1) What is the Euler characteristic of: (i) a line segment; (ii) a
circle?
(2) A disk is the set of points on, or encircled by, a circle in the plane. That
is, if the center is the origin and the radius is r > 0, then the disk of
2radiusr, centered at the origin, is the set of real pairshx,yi∈R such that
2 2 2x +y ≤r . Similarly