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{{Short description|Type of mathematical function}}

[[de:Lokal konstante Funktion]]

{{More citations needed|date=January 2024}}

{{redirect|Locally constant|the sheaf-theoretic term|locally constant sheaf}}

[[File:Example of a locally constant function with sgn(x).svg|thumb|The [[signum function]] restricted to the ___domain <math>\R\setminus\{0\}</math> is locally constant.]]

In [[mathematics]], a '''locally constant function''' is a [[Function (mathematics)|function]] from a [[topological space]] into a [[Set (mathematics)|set]] with the property that around every point of its ___domain, there exists some [[Neighborhood (topology)|neighborhood]] of that point on which it [[Restriction of a function|restricts]] to a [[constant function]].

== Definition ==

In [[mathematics]], a [[function]] ''f'' from a [[topological space]] ''A'' to a [[set]] ''B'' is called '''locally constant''', [[iff]] for every ''a'' in ''A'' there exists a [[neighborhood (topology)|neighborhood]] ''U'' of ''a'', such that ''f'' is constant on ''U''.

Let <math>f : X \to S</math> be a function from a [[topological space]] <math>X</math> into a [[Set (mathematics)|set]] <math>S.</math>

Every constant function is locally constant.

If <math>x \in X</math> then <math>f</math> is said to be '''locally constant at <math>x</math>''' if there exists a [[Neighborhood (topology)|neighborhood]] <math>U \subseteq X</math> of <math>x</math> such that <math>f</math> is constant on <math>U,</math> which by definition means that <math>f(u) = f(v)</math> for all <math>u, v \in U.</math>

The function <math>f : X \to S</math> is called '''locally constant''' if it is locally constant at every point <math>x \in X</math> in its ___domain.

== Examples ==

Every locally constant function from the [[real number]]s '''R''' to '''R''' is constant. But the function ''f'' from the [[rational number|rationals]] '''Q''' to '''R''', defined by ''f''(''x'') = 0 for ''x'' < [[Pi|&pi;]], and ''f''(''x'') = 1 for ''x'' > &pi;, is locally constant (here we use the fact that &pi; is [[irrational number|irrational]] and that therefore the two sets {''x''&isin;'''Q''' : ''x'' < &pi;} and {''x''&isin;'''Q''' : ''x'' > &pi;} are both [[open set|open]] in '''Q'''.

Every [[constant function]] is locally constant. The converse will hold if its [[Domain of a function|___domain]] is a [[connected space]].

Generally speaking, if ''f'' : ''A'' &rarr; ''B'' is locally constant, then it is constant on any [[connected component]] of ''A''. The converse is true for [[locally connected]] spaces (where the connected components are open).

Every locally constant function from the [[real number]]s '''<math>\R'''</math> to '''<math>\R'''</math> is constant, by the [[Connected space|connectedness]] of <math>\R.</math> But the function ''<math>f'' : \Q \to \R</math> from the [[rationalRational number|rationals]] '''<math>\Q'''</math> to '''<math>\R''',</math> defined by ''<math>f''(''x'') = 0 \text{ for ''} x'' < [[Pi|&\pi;]],</math> and ''<math>f''(''x'') = 1 \text{ for ''} x'' > &\pi;,</math> is locally constant (herethis we useuses the fact that &<math>\pi;</math> is [[irrationalIrrational number|irrational]] and that therefore the two sets <math>\{'' x''&isin;''' \in \Q''' : ''x'' < &\pi; \}</math> and <math>\{'' x''&isin;''' \in \Q''' : ''x'' > &\pi; \}</math> are both [[openOpen set|open]] in '''<math>\Q'''</math>).

GenerallyIf speaking, if ''<math>f'' : ''A'' &rarr;\to ''B''</math> is locally constant, then it is constant on any [[Connected space|connected component]] of ''<math>A''.</math> The converse is true for [[locally connected]] spaces, (wherewhich theare spaces whose connected components are open) subsets.

Further examples include the following:

* Given a [[covering map]] ''<math>p'' : ''C'' &rarr;\to ''X'',</math> then to each point ''<math>x'' of\in ''X''</math> we can assign the [[cardinality]] of the [[fibreFiber (mathematics)|fiber]] ''p''<supmath>p^{-1</sup>}(''x'')</math> over ''<math>x''</math>; this assignment is locally constant.

* A map from thea topological space ''<math>A''</math> to a [[discrete space]] ''<math>B''</math> is [[Continuous function (topology)|continuous]] if and only if it is locally constant.

== Connection with sheaf theory ==

There are {{em|sheaves}} of locally constant functions on <math>X.</math> To be more definite, the locally constant integer-valued functions on <math>X</math> form a [[Sheaf (mathematics)|sheaf]] in the sense that for each open set <math>U</math> of <math>X</math> we can form the functions of this kind; and then verify that the sheaf {{em|axioms}} hold for this construction, giving us a sheaf of [[abelian group]]s (even [[commutative ring]]s).<ref>{{cite book |last1=Hartshorne |first1=Robin |title=Algebraic Geometry |date=1977 |publisher=Springer |page=62}}</ref> This sheaf could be written <math>Z_X</math>; described by means of {{em|stalks}} we have stalk <math>Z_x,</math> a copy of <math>Z</math> at <math>x,</math> for each <math>x \in X.</math> This can be referred to a {{em|constant sheaf}}, meaning exactly {{em|sheaf of locally constant functions}} taking their values in the (same) group. The typical sheaf of course is not constant in this way; but the construction is useful in linking up [[sheaf cohomology]] with [[homology theory]], and in logical applications of sheaves. The idea of [[local coefficient system]] is that we can have a theory of sheaves that {{em|locally}} look like such 'harmless' sheaves (near any <math>x</math>), but from a global point of view exhibit some 'twisting'.

== See also ==

* {{annotated link|Liouville's theorem (complex analysis)}}

* [[Locally constant sheaf]]

==References==

{{Reflist}}

{{DEFAULTSORT:Locally Constant Function}}

[[Category:Sheaf theory]]