Namespaces
Variants
Actions

Difference between revisions of "Discrete topology"

From Encyclopedia of Mathematics
Jump to: navigation, search
(Importing text file)
 
(LaTeX)
Line 1: Line 1:
''on a set <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d0331701.png" />''
+
''on a set $X$''
  
The topology in which every set is open (and therefore every set is closed). The discrete topology is the largest element in the lattice of all topologies on the given set. The term  "discrete topology"  is sometimes understood in a somewhat wider sense, viz. as a topology in which intersections of arbitrary numbers of open sets are open. In the case of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d0331702.png" />-spaces, both definitions coincide. In this sense, the theory of discrete spaces is equivalent to the theory of partially ordered sets.
+
The topology in which every set is open (and therefore every set is closed). The discrete topology is the largest element in the lattice of all topologies on the given set. The term  "discrete topology"  is sometimes understood in a somewhat wider sense, viz. as a topology in which intersections of arbitrary numbers of open sets are open. In the case of $T_1$-spaces, both definitions coincide. In this sense, the theory of discrete spaces is equivalent to the theory of partially ordered sets.
  
 
====References====
 
====References====
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  P.S. Aleksandrov,  "Diskrete Räume"  ''Mat. Sb.'' , '''2'''  (1937)  pp. 501–520</TD></TR></table>
+
<table>
 +
<TR><TD valign="top">[1]</TD> <TD valign="top">  P.S. Aleksandrov,  "Diskrete Räume"  ''Mat. Sb.'' , '''2'''  (1937)  pp. 501–520</TD></TR>
 +
</table>
  
  
  
 
====Comments====
 
====Comments====
The equivalence alluded to above is obtained as follows: If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d0331703.png" /> is a pre-ordered set (cf. [[Pre-order|Pre-order]]), then define <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d0331704.png" /> for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d0331705.png" />. With the topology generated by the sets <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d0331706.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d0331707.png" /> becomes a [[Discrete space|discrete space]].
+
The equivalence alluded to above is obtained as follows: If $P$ is a pre-ordered set (cf. [[Pre-order|Pre-order]]), then define $O_x = \{ y \in P : y \le x \}$ for $x \in P$. With the topology generated by the sets $O_x$, $P$ becomes a [[Discrete space|discrete space]].
  
If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d0331708.png" /> is a discrete space, put <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d0331709.png" /> for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d03317010.png" />. Then <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d03317011.png" /> if and only if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d03317012.png" />, defines a pre-order on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d03317013.png" />.
+
If $X$ is a discrete space, put $O_x = \cap \{ O : x \in O, \  O \text open} \}$ for $x \in X$. Then $y \le x$ if and only if $y \in O_x$, defines a pre-order on $X$.
  
These constructions are each others inverses. Moreover, discrete <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d033/d033170/d03317014.png" />-spaces correspond to partial orders and  "real"  discrete spaces correspond to discrete orders.
+
These constructions are each others inverses. Moreover, discrete $T_0$-spaces correspond to partial orders and  "real"  discrete spaces correspond to discrete orders.
  
 
This simple idea and variations thereof have proven to be extremely fruitful, see, e.g., [[#References|[a1]]].
 
This simple idea and variations thereof have proven to be extremely fruitful, see, e.g., [[#References|[a1]]].
  
 
====References====
 
====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  G. Gierz,  K.H. Hofmann,  K. Keimel,  J.D. Lawson,  M.V. Mislove,  D.S. Scott,  "A compendium of continuous lattices" , Springer  (1980)</TD></TR></table>
+
<table>
 +
<TR><TD valign="top">[a1]</TD> <TD valign="top">  G. Gierz,  K.H. Hofmann,  K. Keimel,  J.D. Lawson,  M.V. Mislove,  D.S. Scott,  "A compendium of continuous lattices" , Springer  (1980)</TD></TR>
 +
</table>
 +
 
 +
{{TEX|done}}

Revision as of 20:19, 31 October 2014

on a set $X$

The topology in which every set is open (and therefore every set is closed). The discrete topology is the largest element in the lattice of all topologies on the given set. The term "discrete topology" is sometimes understood in a somewhat wider sense, viz. as a topology in which intersections of arbitrary numbers of open sets are open. In the case of $T_1$-spaces, both definitions coincide. In this sense, the theory of discrete spaces is equivalent to the theory of partially ordered sets.

References

[1] P.S. Aleksandrov, "Diskrete Räume" Mat. Sb. , 2 (1937) pp. 501–520


Comments

The equivalence alluded to above is obtained as follows: If $P$ is a pre-ordered set (cf. Pre-order), then define $O_x = \{ y \in P : y \le x \}$ for $x \in P$. With the topology generated by the sets $O_x$, $P$ becomes a discrete space.

If $X$ is a discrete space, put $O_x = \cap \{ O : x \in O, \ O \text open} \}$ for $x \in X$. Then $y \le x$ if and only if $y \in O_x$, defines a pre-order on $X$.

These constructions are each others inverses. Moreover, discrete $T_0$-spaces correspond to partial orders and "real" discrete spaces correspond to discrete orders.

This simple idea and variations thereof have proven to be extremely fruitful, see, e.g., [a1].

References

[a1] G. Gierz, K.H. Hofmann, K. Keimel, J.D. Lawson, M.V. Mislove, D.S. Scott, "A compendium of continuous lattices" , Springer (1980)
How to Cite This Entry:
Discrete topology. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Discrete_topology&oldid=11525
This article was adapted from an original article by A.A. Mal'tsev (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article