Download A Taste of Topology by Volker Runde (auth.), S Axler, K.A. Ribet (eds.) PDF

By Volker Runde (auth.), S Axler, K.A. Ribet (eds.)

If arithmetic is a language, then taking a topology direction on the undergraduate point is cramming vocabulary and memorizing abnormal verbs: an important, yet no longer consistently fascinating workout one has to head via ahead of it is easy to learn nice works of literature within the unique language.

The current publication grew out of notes for an introductory topology direction on the collage of Alberta. It offers a concise advent to set-theoretic topology (and to a tiny bit of algebraic topology). it really is obtainable to undergraduates from the second one yr on, yet even starting graduate scholars can make the most of a few parts.

Great care has been dedicated to the choice of examples that aren't self-serving, yet already obtainable for college kids who've a historical past in calculus and straightforward algebra, yet now not inevitably in genuine or complicated analysis.

In a few issues, the booklet treats its fabric in a different way than different texts at the subject:

* Baire's theorem is derived from Bourbaki's Mittag-Leffler theorem;

* Nets are used largely, specifically for an intuitive facts of Tychonoff's theorem;

* a brief and chic, yet little recognized evidence for the Stone-Weierstrass theorem is given.

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Example text

5(ii) yields that U is open. As in Euclidean space, we define a set to be closed if its complement is open. 9. Let (X, d) be a metric space. A subset F of X is called closed if X \ F is open. 10. (a) Let (X, d) be any metric space, let x0 ∈ X, and let r > 0. The closed ball centered at x0 with radius r is defined as Br [x0 ] := {x ∈ X : d(x, x0 ) ≤ r}. We claim that Br [x0 ] is indeed closed. To show this, let x ∈ X \ Br [x0 ], that is, such that d(x, x0 ) > r. Let := d(x, x0 )−r > 0, and let y ∈ B (x).

There is x > 0 such that B x (x) contains only finitely many terms of (xn )∞ n=1 ; that is, there is nx ∈ N such that xn ∈ / B x (x) for n ≥ nx . Since {B x (x) : x ∈ K} is an open cover for K, there are x1 , . . , xm ∈ K with K =B x1 (x1 ) ∪ · · · ∪ B xm (xm ). For n ≥ max{nx1 , . . , nxm }, this means that xn ∈ /B x 1 (x1 ) ∪ · · · ∪ B xm (xm ) = K, which is absurd. 8.

N, there is Uj ∈ U such that xj ∈ Uj . It follows that S ⊂ U1 ∪ · · · ∪ Un . Hence, S is compact. (b) Let (X, d) be a compact metric space, and let ∅ = K ⊂ X be compact. Fix x0 ∈ K. Since {Br (x0 ) : r > 0} is an open cover of K, there are r1 , . . , rn > 0 such that K ⊂ Br1 (x0 ) ∪ · · · ∪ Brn (x0 ). With R := max{r1 , . . , rn }, we see that K ⊂ BR (x0 ), so that diam(K) ≤ 2R < ∞. This means, for example, that any unbounded subset of Rn (or, more generally, of any normed space) cannot be compact.

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