## Exercise 4

Solve the following linear equations.

(a) 5

*x*+ 8 = −2(b)

The rational and irrational numbers together make up the **real** numbers. The set of real numbers is denoted by . Like rationals, irrational numbers can be represented by decimals, but unlike the decimals for rational numbers, those for irrationals are neither finite nor recurring. All such infinite non-recurr

In OpenLearn unit M208_5 Mathematical language you met the sets

= {1, 2, 3, …}, the natural numbers;

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*n*th roots o

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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1. Join the 200,000 students currently studying with The Open Unive

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):

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.

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.

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*

*We check that T_{f} satisfies conditions (T1)–(T3) for a topology.*

*
*

*Since (T1)–(T3) are satisfied, T_{f} is a topology on I(X).*

*Thus ( I(X),T_{f}) is a topological space. We give the topology T_{f} a sp*

*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 *

*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*

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.

*It follows from the Author(s):*

*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 β, its orientability number ω and its Euler characteristic χ, each of which is a topological invariant – 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

*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 −1.*

*Author(s):*

*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 −1, and so on.*

*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*

*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 ‘3-manifolds’, topological s*

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

*Definition*

*The Euler characteristic χ of a subdivision of a surface is*

*
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