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MCS 451-Exercises Q1. Show that every closed subset of a compact space is compact. Q2. Let π― and π― β be topologies on π with π― β π― β . a) Show that if π― β is compact then π― is compact b) Is the converse true? Namely, if π― is compact then can we conclude that π― β is compact? c) Prove that if π― and π― β are both compact and Hausdorο¬, then π― = π― β Q3. Let π be a topological space and π΄, π΅ β π compact subspaces. (1) Is π΄ βͺ π΅ compact? (2) Is π΄ β© π΅ compact? (3) The same question under the assumption that π is Hausdorο¬. Q4. Let π be Hausdorο¬ and π΄ β π dense and locally compact. Prove that π΄ is open. Deduce from this that any locally compact subspace of a Hausdorο¬ space is relatively closed, (i.e. open in its closure). Q5. Show that the rationals β are not locally compact. Proof: Let π be any rational, with πΎ a compact set containing π. Any neighborhood of π will contain an interval (π, π) β© β around π, and this neigh borhood, when considered as a subset of β must contain some irrational π‘. It is straightforward to construct a sequence of rationals in (π, π) β© β which converge to π‘ β β and, as a subset of the rationals, clearly have no (rational) limit point. It follows that (π, π) β© β cannot be in πΎ as (π, π) contains an inο¬nite sequence with no convergent subsequence. Hence β is not locally compact at π (or any rational). Q6. Let π be an uncountable set, and let π΄ be a countable subset of π. Show that π β π΄ is uncountable. Q7. Consider the set π = {π, π, π}, and the collection of subsets π― = {β , π, {π}, {π, π}}. List the compact subsets of π. Give an example of a function π : π β β that is continuous, and one example of a function π : π β β that is not. Q8. Give an example of a continuous function on (0, 1) which is bounded but attains neither a maximum value nor a minimum value. Q9. Show that a closed subspace of a normal space is normal. Q10. Show that a closed subspace of a normal space is normal. Q11. Show that every locally compact Hausdorο¬ space is regular. 1