## Exercise 4

Solve the following linear equations.

(a)Â Â 5

*x*+ 8 = âˆ’2(b)Â Â

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

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)â€“(T3) for a topology.*

*
*

*Since (T1)â€“(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*

By â€˜containsâ€™, we mean that we can find part of the surface that is homeomorphic to a MÃ¶bius band. The edge of the MÃ¶bius band does not need to correspond to an edge at the surface, so that a surface without boundary can be non-orientable (as we shall shortly see).

When seeking MÃ¶bius bands in a surface, it can be helpful to look at all possible closed curves on the surface and thicken these into bands.

Remember, fro

This theorem applies to

*all*surfaces and not just to surfaces in space.This theorem tells us that the boundary number is a

**topological invariant**for surfaces, i.e. a property that is invariant under homeomorphisms.It follows from the theorem that two surfaces with different boundary numbers cannot be homeomorphic. It does not follow that two surfaces with the same boundary number

*are*homeomorphic â€“

*Surfaces can be constructed in a similar way from plane figures other than polygons. For example, starting with a disc, we can fold the left-hand half over onto the right-hand half, and identify the edges labelled a, as shown in Figure 36; this is rather like zipping up a purse, or â€˜crimpingâ€™ a Cornish pasti*

*As the polygons become more complicated, so the identifications become more difficult to visualise. For example, what happens if we try to identify the edges of an octagon in pairs, as indicated by the edge labels and arrowheads in Figure 34? Author(s): *

*There are two other surfaces that can be obtained by identifying both pairs of opposite edges of a rectangle. In one of these, shown in Figure 31, we first identify the edges AB and A'B', labelled a, in the direction shown by the arrowheads. This gives us a cylinder, as before. We then try to ident*

*In Section 2 we start by introducing surfaces informally, considering several familiar examples such as the sphere, cube and MÃ¶bius band. We also illustrate how surfaces can be constructed from a polygon by identifying edges. A more formal approach to surfaces is presented at the end of the section.*

*Figure 3 shows*

*By the end of this unit you should be able to:*

*explain the terms**surface*,*surface in space*,*disc-like neighbourhood*and*half-disc-like neighbourhood*;*explain the terms**n-fold torus*,*torus with n holes*,*MÃ¶bius band*and*Klein bottle*;*explain what is meant by the**boundary*of a surface, and determine the*boundary number*of a given surface with boundary;*construct certa*

*This unit is concerned with a special class of topological spaces called surfaces. Common examples of surfaces are the sphere and the cylinder; less common, though probably still familiar, are the torus and the MÃ¶bius band. Other surfaces, such as the projective plane and the Klein bottle, may be unfamiliar, but they crop up in many places in mathematics. Our aim is to classify surfaces â€“ that is, to produce criteria that allow us to determine whether two given surfaces are h*

*The content acknowledged below is Proprietary (see terms and conditions) and is used under licence.*

All materials included in this unit are derived from content originated at the Open University.

*Modelling pollution in the Great Lakes: a review *

*The main teaching text of this unit is provided in the workbook below. The answers to the exercises that you'll find throughout the workbook are given in the answer book. You can access it by clicking on the link under the workbook. When prompted to watch the video for this unit, return to this page and watch the clips below. After you've watched the clips, return to the workbook.*

*Click 'View document' to open the workbook (PDF, 0.3 MB).*

*This is the fifth and final unit in the MSXR209 series on mathematical modelling. In this unit we revisit the model developed in the first unit of this series on pollution in the Great Lakes of North America. Here we evaluate and revise the original model by comparing its predictions against data from the lakes before finally reflecting on the techniques used.*

*This unit, the fifth in a series of five, builds on ideas developed and introduced in Modelling pollution in the Great Lakes *

**Studying mammals: The opportunists Many mammals are food specialists, with complex adaptations that gear them toward a particular food source. So how do the omnivores survive and prosper without these fancy evolutionary features? This free course, Studying mammals: The opportunists, examines the physiology, diet and strategies of some of these opportunistic feeders. It is the sixth course in the Studying mammals series.Author(s): **

**Studying mammals: Return to the water Some of the most unusual and versatile of all the mammals are the groups that live, feed and reproduce underwater. In this free course, Studying mammals: Return to the water, we will see how these formerly land-based mammals adapted to a return to the water, discussing such challenges as breathing, movement and communication. This is the seventh course in the Studying mammals series.Author(s): **

**Neighbourhood nature There is a fascinating world of nature all around us which we can see if we know how to look for it. Wherever you live, be it in a city or the countryside, you will find areas that support a range of wildlife. This free course, Neighbourhood nature, will provide you with basic scientific and observational skills so that you can go into your local neighbourhood to discover the animals and plants in open spaces. You will learn how to observe, identify and record the wildlife around you, building u**

**
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