In OpenLearn unit M208_5 Mathematical language you met the sets

• = {1, 2, 3, â€¦}, the natural numbers;

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After studying this unit you should be able to:

• understand the arithmetical properties of the rational and real numbers;

• understand the definition of a complex number;

• perform arithmetical operations with complex numbers;

• represent complex numbers as points in the complex plane;

• determine the polar form of a complex number;

• use de Moivre's Theorem to find the nth roots o
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In this unit we look at some different systems of numbers, and the rules for combining numbers in these systems. For each system we consider the question of which elements have additive and/or multiplicative inverses in the system. We look at solving certain equations in the system, such as linear, quadratic and other polynomial equations.

In Section 1 we start by revising the notation used for the rational numbers and the real numbers, and we list their arithmetical prop
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In our last example, we consider a pentagon with two pairs of edges identified. As we saw in Section 2.3, identification of the edges produces a torus with a hole. In this case there are five vertex-neighbourhoods to fit together, as shown in Author(s): The Open University

If x lies on an edge, then each of the two points in [x] has a half-disc-like neighbourhood (see Figure 107). When we identify edges, these neighbourhoods fit together to form disc-like neighbourhoods in the Klein bottle.

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If x lies on an edge, then each of the two points in [x] has a half-disc-like neighbourhood. When we identify edges, these neighbourhoods fit together to form disc-like neighbourhoods on the torus, as Figure 105 shows.

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We know that a polygon X is a surface and so each point x in X has a disc-like or half-disc-like neighbourhood. We shall show that a map f that identifies edges of a polygon to create an object Y automatically creates corresponding disc-like or half-disc-like neighbourhoods of each point y = f(x) of Y.

If x is in the interior of X, there is no difficulty: the point x has a disc-like neighbourhood U
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5.2.1 Proof

We check that Tf satisfies conditions (T1)â€“(T3) for a topology.

Since (T1)â€“(T3) are satisfied, Tf is a topology on I(X).

Thus (I(X),Tf) is a topological space. We give the topology Tf a sp
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5.2 The identification topology

Our aim is to show that the object that we produce when we identify some or all the edges of a polygon is a surface. Therefore, by the definition of a surface given in Section 2.5, we must show how it can be given the structure of a topological space, and that this space is Hausdorff. Furthermore, we must show that every point has
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5.1 Identifying edges of a polygon

In this section, we revisit the construction of surfaces by identifying edges of polygons, as described in Section 2. Recall that, if we take any polygon in the plane and identify some of its edges in pairs, then we obtain a surface. When specifying how a given pair of edges is to be identified, we choose one of the two possible re
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4.6.1 Remarks

1. We already know that the characteristic numbers are topological invariants, that is, any two homeomorphic surfaces have the same values for the characteristic numbers. Thus it is solely the converse, namely if two surfaces have the same values for the characteristic numbers then they are homeomorphic, that we have to prove.

2. It follows from the Author(s): The Open University

4.6 The Classification Theorem

In this subsection we state the Classification Theorem for surfaces, which classifies a surface in terms of its boundary number Î², its orientability number Ï‰ and its Euler characteristic Ï‡, each of which is a topological invariant â€“ it is preserved under homeomorphisms.

Let us remind ourselves of these three numbers.

• A surface may or may not have a boundary, and, if it does, then the boundary has finitely many disjoint pieces. The nu
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4.5.2 n-fold toruses

We can use a similar technique to find the Euler characteristic of a 2-fold torus. If we cut the surface into two, as shown in Figure 95, and separate the pieces, we obtain two copies of a 1-fold torus with 1 hole, each with Euler characteristic âˆ’1.

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4.5.1 Surfaces with holes

Using this result, we can obtain the Euler characteristic of a surface with any number of holes by successively inserting the holes one at a time. For example, since a closed disc has Euler characteristic 1, it follows that a closed disc with 1 hole has Euler characteristic 0, a disc with 2 holes has Euler characteristic âˆ’1, and so on.

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4.5 Some general results

We next establish some general results about Euler characteristics. We start with a theorem that tells us what happens to the Euler characteristic of a surface when we remove an open disc.

## Theorem 10: Disc Removal Theorem

The Euler characteristic of a surface with an open disc removed is one le
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4.4 Historical note on the Euler characteristic

A little history is instructive here, because it shows how difficult Theorem 9 really is. By 1900 the classification of compact surfaces was well understood, although proofs of the major theorems relied more on intuition than would be acceptable today. Attention switched to objects called â€˜3-manifoldsâ€™, topological s
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4.3 The Euler characteristic

Subdivisions of surfaces lead to the third number used to classify surfaces, the Euler characteristic.

## Definition

The Euler characteristic Ï‡ of a subdivision of a surface is

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4.2 Subdivisions

In this subsection we formalise the idea of a net by introducing a useful concept called a subdivision of a surface. This is a standard kind of net drawn on a surface, and is defined in terms of vertices, edges and faces. It leads to the idea of the Euler characteristic of the surface.

All surfaces obtained from polygons by identifying edges arise from a net (of sorts) consisting of a single polygonal face, together with the edges and vertices that remain aft
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4.1 Nets on surfaces

In Section 4 we introduce the third of the numbers we associate with a surface â€“ the Euler characteristic. This is used in the Classification Theorem, which we state at the end of the section. To define the Euler characteristic, we need the idea of a subdivision of a surface, which we introduce by first c
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