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Inside PFTB Proofs from The Book is indeed a glimpse of mathematical heaven, where clever insights and beautiful ideas combine in astonishing and glorious ways. There is vast wealth within its pages, one gem after another. Some of the proofs are classics, but many are new and brilliant proofs of classical results. Suppose that.

## Proofs From The Book

In their work on the distribution of roots of algebraic equations, Littlewood and Offord proved in the following result:. What is the most interesting formula involving elementary functions? Suppose that you drop a short needle on ruled paper — what is then the probability that the needle comes to lie in a position where it crosses one of the lines? Some mathematical principles, such as the two in the title of this chapter, are so obvious that you might think they would only produce equally obvious results.

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We will encounter instances of them also in later chapters. Some mathematical theorems exhibit a special feature: The statement of the theorem is elementary and easy, but to prove it can turn out to be a tantalizing task — unless you open some magic door and everything becomes clear and simple. One such example is the following result due to Nicolaas de Bruijn:. Whenever a rectangle is tiled by rectangles all of which have at least one side of integer length, then the tiled rectangle has at least one side of integer length.

In this chapter we are concerned with a basic theme of combinatorics: properties and sizes of special families.

## Proofs from The Book | AMS Grad Blog

These two results have in common that they were reproved many times and that each of them initiated a new field of combinatorial set theory. For both theorems, induction seems to be the natural method, but the arguments we are going to discuss are quite different and truly inspired. Of course, getting meaningful answers to such problems heavily depends on formulating meaningful questions. For the card shuffling problem, this means that we have. The essence of mathematics is proving theorems — and so, that is what mathematicians do: They prove theorems. But to tell the truth, what they really want to prove, once in their lifetime, is a.

Now what makes a mathematical statement a true Lemma? First, it should be applicable to a wide variety of instances, even seemingly unrelated problems.

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Secondly, the statement should, once you have seen it, be completely obvious. And thirdly, on an esthetic level, the Lemma — including its proof — should be beautiful! One of the most beautiful formulas in enumerative combinatorics concerns the number of labeled trees. Consider the set. It gained immediate prominence and, together with its higher-dimensional analogs, helped initiate a whole new field, today called.

### Summer 2003 `B' Term-- Proofs from the Book -- MAT 5932-24

Any such motion takes place in a compact subset of the plane. Some of the oldest combinatorial objects, whose study apparently goes back to ancient times, are the. Here is the problem we want to discuss. At some point he stops and asks us to fill in the remaining cells so that we get a Latin square. When is this possible? In order to have a chance at all we must, of course, assume that at the start of our task any element appears at most once in every row and in every column.

Let us give this situation a name. We speak of a. The four-color problem was a main driving force for the development of graph theory as we know it today, and coloring is still a topic that many graph theorists like best. Here is a simple-sounding coloring problem, raised by Jeff Dinitz in , which defied all attacks until its astonishingly simple solution by Fred Galvin fifteen years later. If in the usual representation of the determinant we forget about the signs of the permutations we get the permanent. Plane graphs and their colorings have been the subject of intensive research since the beginnings of graph theory because of their connection to the fourcolor problem.

As stated originally the four-color problem asked whether it is always possible to color the regions of a plane map with four colors such that regions which share a common boundary and not just a point receive different colors. The figure on the right shows that coloring the regions of a map is really the same task as coloring the vertices of a plane graph. As in Chapter 12 page 75 place a vertex in the interior of each region including the outer region and connect two such vertices belonging to neighboring regions by an edge through the common boundary.

Here is an appealing problem which was raised by Victor Klee in Suppose the manager of a museum wants to make sure that at all times every point of the museum is watched by a guard. The guards are stationed at fixed posts, but they are able to turn around. How many guards are needed? In fact, the guard may be stationed at any point of the museum. But, in general, the walls of the museum may have the shape of any closed polygon.