Namespaces
Variants
Actions

Difference between revisions of "Specialization of a point"

From Encyclopedia of Mathematics
Jump to: navigation, search
(better)
(Characterisation of a T0 space)
Line 16: Line 16:
  
 
====Comments====
 
====Comments====
Of course, $\overline{\{x\}}$ denotes the closure of the [[singleton]] set $\{x\}$. The closure of a point is an [[Irreducible topological space|irreducible]] subset of $X$, and conversely, every irreducible subset of $X$ has a generic point.
+
Here $\overline{\{x\}}$ denotes the [[Closure of a set|closure]] of the [[singleton]] set $\{x\}$. The closure of a point is an [[Irreducible topological space|irreducible]] subset of $X$, and conversely, every irreducible subset of $X$ has a generic point.
  
 
====References====
 
====References====
Line 23: Line 23:
 
</table>
 
</table>
  
 +
====Comments====
 +
The relation "$y$ is a specialisation of $x$" on $X$, denoted $y \sqsupseteq x$, is reflexive and transitive.  It is anti-symmetric, and hence a [[partial order]] on $X$, if and only if, $X$ is a [[T0 space]].
 +
 +
====References====
 +
<table>
 +
<TR><TD valign="top">[b1]</TD> <TD valign="top"> Steven Vickers ''Topology via Logic'' Cambridge Tracts in Theoretical Computer Science '''5''' Cambridge University Press (1989) ISBN 0-521-36062-5 {{ZBL|0668.54001}} </TD></TR>
 +
</table>
 
{{TEX|done}}
 
{{TEX|done}}

Revision as of 17:24, 1 January 2016

$x$ in a topological space $X$

A point $y \in X$ for which the inclusion $y \in \overline{\{x\}}$ holds; equivalently the inclusion $\overline{\{y\}} \subseteq \overline{\{x\}}$; every neighbourhood of $x$ is a neighbourhood of $y$.

A point $x$ is called generic if any point of $X$ is a specialization of it, that is, if $\overline{\{x\}} = X$. The other extreme case is that of a closed point: a point which has a unique specialization, namely the point itself.

For the affine scheme $\mathrm{Spec}(A)$ of a ring $A$, a point $y$ is a specialization of a point $x$ if for the corresponding prime ideals of $A$ the inclusion $\mathfrak{p}_x \subseteq \mathfrak{p}_y$ holds. When $A$ is a ring without zero divisors, the point $\{0\}$ is the generic one. The relation of specialization distributes into levels: the highest are the closed points, on the next level are the points whose specializations are closed, and on the $i$-th level are the points whose specializations belong to the levels with labels $\le i-1$. For example, for $\mathrm{Spec}(\mathbf{C}[T_1,\ldots,T_n]$ there are $n+1$ levels: closed points, generic points of curves, generic points of surfaces,$\ldots$, the generic point of the $n$-dimensional affine space.

References

[1] Yu.I. Manin, "Lectures on algebraic geometry" , 1 , Moscow (1970) (In Russian) MR0284434 Zbl 0204.21302
[2] A. Grothendieck, J. Dieudonné, "Eléments de géometrie algébrique" , I. Le langage des schémes , Springer (1971) MR0217085 Zbl 0203.23301


Comments

Here $\overline{\{x\}}$ denotes the closure of the singleton set $\{x\}$. The closure of a point is an irreducible subset of $X$, and conversely, every irreducible subset of $X$ has a generic point.

References

[a1] R. Hartshorne, "Algebraic geometry" , Springer (1977) pp. Sect. IV.2 MR0463157 Zbl 0367.14001

Comments

The relation "$y$ is a specialisation of $x$" on $X$, denoted $y \sqsupseteq x$, is reflexive and transitive. It is anti-symmetric, and hence a partial order on $X$, if and only if, $X$ is a T0 space.

References

[b1] Steven Vickers Topology via Logic Cambridge Tracts in Theoretical Computer Science 5 Cambridge University Press (1989) ISBN 0-521-36062-5 Zbl 0668.54001
How to Cite This Entry:
Specialization of a point. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Specialization_of_a_point&oldid=37222
This article was adapted from an original article by V.V. Shokurov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article