Restriction (mathematics)
Function  

x ↦ f (x)  
Examples by domain and codomain  


Classes/properties  
Constant · Identity · Linear · Polynomial · Rational · Algebraic · Analytic · Smooth · Continuous · Measurable · Injective · Surjective · Bijective  
Constructions  
Restriction · Composition · λ · Inverse  
Generalizations  
Partial · Multivalued · Implicit  
In mathematics, the restriction of a function f is a new function obtained by choosing a smaller domain A for the original function . The notation is also used.
Contents
Formal definition[edit]
Let be a function from a set E to a set F. If a set A is a subset of E, then the restriction of to is the function^{[1]}
given by f_{A}(x) = f(x) for x in A. Informally, the restriction of f to A is the same function as f, but is only defined on .
If the function f is thought of as a relation on the Cartesian product , then the restriction of f to A can be represented by the graph , where the pairs represent edges in the graph G.
Examples[edit]
 The restriction of the noninjective function to the domain is the injection.
 The factorial function is the restriction of the gamma function to the positive integers, with the argument shifted by one:
Properties of restrictions[edit]
 Restricting a function to its entire domain gives back the original function, i.e., .
 Restricting a function twice is the same as restricting it once, i.e. if , then .
 The restriction of the identity function on a set X to a subset A of X is just the inclusion map from A into X.^{[2]}
 The restriction of a continuous function is continuous.^{[3]}^{[4]}
Applications[edit]
Inverse functions[edit]
For a function to have an inverse, it must be onetoone. If a function f is not onetoone, it may be possible to define a partial inverse of f by restricting the domain. For example, the function
defined on the whole of is not onetoone since x^{2} = (−x)^{2} for any x in . However, the function becomes onetoone if we restrict to the domain , in which case
(If we instead restrict to the domain , then the inverse is the negative of the square root of y.) Alternatively, there is no need to restrict the domain if we don't mind the inverse being a multivalued function.
Selection operators[edit]
In relational algebra, a selection (sometimes called a restriction to avoid confusion with SQL's use of SELECT) is a unary operation written as or where:
 and are attribute names,
 is a binary operation in the set ,
 is a value constant,
 is a relation.
The selection selects all those tuples in for which holds between the and the attribute.
The selection selects all those tuples in for which holds between the attribute and the value .
Thus, the selection operator restricts to a subset of the entire database.
The pasting lemma[edit]
The pasting lemma is a result in topology that relates the continuity of a function with the continuity of its restrictions to subsets.
Let be two closed subsets (or two open subsets) of a topological space such that , and let also be a topological space. If is continuous when restricted to both and , then is continuous.
This result allows one to take two continuous functions defined on closed (or open) subsets of a topological space and create a new one.
Sheaves[edit]
Sheaves provide a way of generalizing restrictions to objects besides functions.
In sheaf theory, one assigns an object in a category to each open set of a topological space, and requires that the objects satisfy certain conditions. The most important condition is that there are restriction morphisms between every pair of objects associated to nested open sets; i.e., if , then there is a morphism res_{V,U} : F(U) → F(V) satisfying the following properties, which are designed to mimic the restriction of a function:
 For every open set U of X, the restriction morphism res_{U,U} : F(U) → F(U) is the identity morphism on F(U).
 If we have three open sets W ⊆ V ⊆ U, then the composite res_{W,V} o res_{V,U} = res_{W,U}.
 (Locality) If (U_{i}) is an open covering of an open set U, and if s,t ∈ F(U) are such that s_{Ui} = t_{Ui} for each set U_{i} of the covering, then s = t; and
 (Gluing) If (U_{i}) is an open covering of an open set U, and if for each i a section s_{i} ∈ F(U_{i}) is given such that for each pair U_{i},U_{j} of the covering sets the restrictions of s_{i} and s_{j} agree on the overlaps: s_{i}_{Ui∩Uj} = s_{j}_{Ui∩Uj}, then there is a section s ∈ F(U) such that s_{Ui} = s_{i} for each i.
The collection of all such objects is called a sheaf. If only the first two properties are satisfied, it is a presheaf.
Left and rightrestriction[edit]
More generally, the restriction (or domain restriction or leftrestriction) A ◁ R of a binary relation R between E and F may be defined as a relation having domain A, codomain F and graph G(A ◁ R) = {(x, y) ∈ G(R)  x ∈ A} . Similarly, one can define a rightrestriction or range restriction R ▷ B. Indeed, one could define a restriction to nary relations, as well as to subsets understood as relations, such as ones of E × F for binary relations. These cases do not fit into the scheme of sheaves.^{[clarification needed]}
Antirestriction[edit]
The domain antirestriction (or domain subtraction) of a function or binary relation R (with domain E and codomain F) by a set A may be defined as (E \ A) ◁ R; it removes all elements of A from the domain E. It is sometimes denoted A ⩤ R.^{[5]} Similarly, the range antirestriction (or range subtraction) of a function or binary relation R by a set B is defined as R ▷ (F \ B); it removes all elements of B from the codomain F. It is sometimes denoted R ⩥ B.
See also[edit]
 Constraint
 Deformation retract
 Function (mathematics) § Restriction and extension
 Binary relation § Restriction
 Relational algebra § Selection (σ)
References[edit]
 ^ Stoll, Robert (1974). Sets, Logic and Axiomatic Theories (2nd ed.). San Francisco: W. H. Freeman and Company. p. 5. ISBN 0716704579.
 ^ Halmos, Paul (1960). Naive Set Theory. Princeton, NJ: D. Van Nostrand. Reprinted by SpringerVerlag, New York, 1974. ISBN 0387900926 (SpringerVerlag edition). Reprinted by Martino Fine Books, 2011. ISBN 9781614271314 (Paperback edition).
 ^ Munkres, James R. (2000). Topology (2nd ed.). Upper Saddle River: Prentice Hall. ISBN 0131816292.
 ^ Adams, Colin Conrad; Franzosa, Robert David (2008). Introduction to Topology: Pure and Applied. Pearson Prentice Hall. ISBN 9780131848696.
 ^ Dunne, S. and Stoddart, Bill Unifying Theories of Programming: First International Symposium, UTP 2006, Walworth Castle, County Durham, UK, February 57, 2006, Revised Selected ... Computer Science and General Issues). Springer (2006)