T1 not implies Hausdorff - Revision history

Vipul at 18:53, 26 October 2009

2009-10-26T18:53:13Z

← Older revision		Revision as of 18:53, 26 October 2009
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	A [[T1 space]] need not be a [[Hausdorff space]].		A [[T1 space]] need not be a [[Hausdorff space]].

			==Related facts==

			* [[T1 not implies US]]
			* [[US not implies Hausdorff]]

	==Proof==		==Proof==

Vipul at 18:52, 26 October 2009

2009-10-26T18:52:45Z

← Older revision		Revision as of 18:52, 26 October 2009
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	==Proof==		==Proof==

	===Example of ~~the~~ cofinite topology===		===Example of cofinite topology===

	Consider a countable set, say the set <math>\{ 1,2,3, \dots \}</math> of natural numbers, equipped with the [[cofinite topology]]. In the cofinite topology, the nonempty open subsets are precisely the cofinite subsets -- the subsets whose complement is finite. With this topology:		Consider a countable set, say the set <math>\{ 1,2,3, \dots \}</math> of natural numbers, equipped with the [[cofinite topology]]. In the cofinite topology, the nonempty open subsets are precisely the cofinite subsets -- the subsets whose complement is finite. With this topology:

	* The space is <math>T_1</math>: Every point is closed, because its complement is an open subset. Equivalently, if <math>x,y</math> are two natural numbers, the complement of <math>x</math> is an open subset containing <math>y</math> but not <math>x</math>.		* The space is <math>T_1</math>: Every point is closed, because its complement is an open subset. Equivalently, if <math>x,y</math> are two natural numbers, the complement of <math>x</math> is an open subset containing <math>y</math> but not <math>x</math>.
	* The space is not Hausdorff: it is not possible to separate two points with disjoint open subsets, because any two nonempty open subsets have an infinite intersection. This is because the union of their complements is a union of two finite subsets, and hence is not the whole space.		* The space is not Hausdorff: it is not possible to separate two points with disjoint open subsets, because any two nonempty open subsets have an infinite intersection. This is because the union of their complements is a union of two finite subsets, and hence is not the whole space. (Another way of phrasing this is that the space is an [[irreducible space]] and hence, since it has more than one point, it is not a [[Hausdorff space]]).

			===Example of line with two origins===

			{{further\|[[particular example::line with two origins]]}}

			Consider the line with two origins. This is the real line with two copies of the point <math>0</math>. Alternatively, it is the quotient of two copies of the real line where we identify all the nonzero points on the first real line with the corresponding nonzero point on the second real line.

			This is a <math>T_1</math> space, but is not Hausdorff, because the two origins cannot be separated by disjoint open subsets.

Vipul: Created page with '{{topospace property non-implication| stronger = T1 space| weaker = Hausdorff space}} ==Statement== A T1 space need not be a Hausdorff space. ==Proof== ===Example of …'

2009-10-26T18:39:24Z

Created page with '{{topospace property non-implication| stronger = T1 space| weaker = Hausdorff space}} ==Statement== A T1 space need not be a Hausdorff space. ==Proof== ===Example of …'

New page

{{topospace property non-implication|
stronger = T1 space|
weaker = Hausdorff space}}

==Statement==

A [[T1 space]] need not be a [[Hausdorff space]].

==Proof==

===Example of the cofinite topology===

Consider a countable set, say the set <math>\{ 1,2,3, \dots \}</math> of natural numbers, equipped with the [[cofinite topology]]. In the cofinite topology, the nonempty open subsets are precisely the cofinite subsets -- the subsets whose complement is finite. With this topology:

* The space is <math>T_1</math>: Every point is closed, because its complement is an open subset. Equivalently, if <math>x,y</math> are two natural numbers, the complement of <math>x</math> is an open subset containing <math>y</math> but not <math>x</math>.
* The space is not Hausdorff: it is not possible to separate two points with disjoint open subsets, because any two nonempty open subsets have an infinite intersection. This is because the union of their complements is a union of two finite subsets, and hence is not the whole space.