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# Hausdorff and Non-Hausdorff Spaces

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Hausdorff and Non-Hausdorff Spaces

Hausdorff and Non-Hausdorff Spaces

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• 1. Hausdorff and Non-Hausdorff Spaces Mayra Ibarra MATH101
• 2. What is a Topological Space?
• Recall:
• A topological space is an ordered pair (X, Ƭ), where X is a set and T is a collection of subsets of X such that
• Ø Є Ƭ and X Є Ƭ
• U ∩V Є Ƭ whenever U, V Є Ƭ ;
• ∪ { U a : a Є I } Є Ƭ whenever {U a : a Є I } ⊂ Ƭ. 1
• 3. Separation Axioms 2
• T 0 : If a, b ϵ X, there exists an open set O ϵ Ƭ such that either a ϵ O and b O, or b ϵ O and a O.
• T 1 : If a, b ϵ X, there exist open sets O a , O b ϵ Ƭ containing a and b respectively, such that b O a and a O b .
• 4. T 2 : If a, b ϵ X, there exist disjoint open sets O a and O b containing a and b respectively.
• 5.
• 6. T 3 : If A is a closed set and b is a point not in A, there exist disjoint open sets O A and O b containing A and b respectively. X
• 7. T 4 : If A and B are disjoint closed sets in X, there exist disjoint open sets O A and O B containing A and B respectively X
• 8. T 5 : If A and B are separated sets in X, there exist disjoint open sets O A and O B containing A and B respectively
• 9. Hausdorff Spaces
• All metric spaces are Hausdorff 3
• The real number line with the usual topology is Hausdorff 4
• Suppose we have an open set, S, of real numbers
• For each p Є S we can find an ε >0, such that the ε -neighborhood of p forms an open set inside of S.
• Suppose p, q Є S and are separated by a distance r. We can pick ε 1 and ε 2 for p and q , respectively, such that r/2 ≥ ε 1 > 0 and r/2 ≥ ε 2 > 0. Thus p and q will be separated by disjoint open sets.
• 10. Subspaces of Hausdorff Spaces are also Hausdorff 5
• If Y ⊂ X and (X, d) is Hausdorff, then any two distinct points x 1 , x 2 Є Y will also be in X. Since x 1 and x 2 are in X, there are disjoint open sets O 1 and O 2 containing x 1 and x 2 , respectively. Thus Y contains points that can be separated by disjoint open sets and is Hausdorff.
• 11. Non-Hausdorff Spaces
• Zariski Topology
• Let X be [0, 1]∪[2, 3]. Define the following equivalence relation on X:
• a ~ a + 2 for all 0 ≤ a ≤ 1 except for a = ½
• b ~ b – 2 for 2 ≤ b ≤ 3 except for b = 5/2
[0] [1] 5/2 1/2
• 12. Proof:
• Notice that every point in [0, 1] is identified with a point of [2, 3] except fpr the two points ½ and 5/2, which remain unidentified. Thus are contained in X/~. A set in X/~ is open iff it is open in X. However, we see that there are no disjoint open sets that contain ½ and 5/2. So X is Non-Hausdorff when considered under this equivalence relation.6
• 13. Regular and Normal Spaces 7
• Regular Space 8- A space which is both a T 1 and a T 3 space
Normal Space 9 -A space which is both a T 1 and a T 4 space
• 14.
• Every Normal Space is Regular
• Every Regular Space is Hausdorff
• Not all Hausdorff Spaces are Regular
• In the definitions of Normal and Regular we can replace the use of disjoint open sets, and use disjoint closures to yield the same spaces
• The same is not true for the definition of Hausdorff
• 15. Completely Hausdorff 10
• T 2 ½ Axiom- If a and b are two points of a topological space X, there exist open sets O a and O b containing a and b, respectively, such that Ō a ∩ Ō b = Ø
• A space that satisfies this axiom is called a completely Hausdorff space.
• 16.
• 17. Compact Hausdorff Spaces
• If a Hausdorff space is compact, then it is normal, and therefore also regular.
• 18.
• Let the sequence (S n ) in a Hausdorff Space have a limit, s. Suppose that the sequence has another limit, p, such that p≠s. Let U and V be disjoint neighborhoods of s and p, respectively. Then if we choose a sufficiently large n, S n Є U however, for a sufficiently large n, S n Є V. This contradicts the Hausdorff property because U ∩V ≠ Ø. Thus the sequence cannot have more than one limit.
• 19. If X is Hausdorff and (S n ) is a sequence in X that converges to a point s Є X, and if y is an accumulation point of the set {S n | n = 1, 2, . . .}, then s = y.
• 20.
• Suppose s≠y. Then there exist open sets U, V Є X for s and y respectively such that U∩V = Ø. Also, since (S n ) converges to s, there exists a natural number, N such that n>N implies that S n Є U.
• Let i be such that 1< i < N, and let W i , V i Є X be open sets for s i and y, respectively, such that W i ∩ V i = Ø, unless y= s i for some i.
• Define V i ’= V i in case y≠s i , and V i ’= V in case y=s i and define V’ as the intersection of V with a finite collection of open sets, V i ’. So y Є V’ Є a system of neighborhoods around y.
• 21.
• Let z be such that z Є V’ and y≠z, then z≠s i for any i since if i > N, s i Є U and U∩V’ ⊆ U∩V = Ø,
• And if i ≤ N, then either y=s i ≠z, or s i Є W i , and W i ∩V’ ⊆ W i ∩ V i = Ø.
• Thus there is a neighborhood V’ of y, such that no point z ≠ y of the set {S n | n = 1, 2, . . .}, belongs to V’.
• This contradicts the fact that y is an accumulation point of the set {S n | n = 1, 2, . . .},
• thus s=y.
• 22. Endnotes
• Greever, John. Theory and Examples of Point-Set Topology. Claremont: Waybread Publications, 1990.
• Steen, Lynn Arthur and J. Arthur Seebach, Jr. Counterexamples in Topology. New York: Dover Publications Inc., 1995.
• Image of axiom spaces: http://jtauber.com/2005/01/separation.png
• Sneddon, I. N. Ed. Andrew H. Wallace. “An Introduction To Algebraic Topology.” International Series of Monographs in Pure and Applied Mathematics. V.I. New York: Pergamon Press, 1957.
• Baum, John D. Elements of Point Set Topology . New York: Dover Publications Inc., 1991
• Sneddon, pg. 32.
• Goodman, Sue E. Beginning Topology. Belmont: Brooks/Cole, 2005.
• Baum, pg. 81.
• Image-http://en.wikipedia.org/wiki/Regular_space
• Image-http://commons.wikimedia.org/wiki/Image:Normal_space.svg
• Steen, pg. 13.
• Steen, pg. 13.
• Sneddon, pg. 31.