Injective function

From Wikipedia, the free encyclopedia

In mathematics, an injective function is a function which associates distinct arguments to distinct values. More precisely, a function f is said to be injective if it maps distinct x in the domain to distinct y in the codomain, such that f(x) = y.

Put another way, f is injective if f(a) = f(b) implies a = b (or a b implies f(a)  f(b)), for any a, b in the domain.

An injective function is called an injection, and is also said to be an information-preserving or one-to-one function. (However, this name is best avoided, since some authors understand it to mean a one-to-one correspondence, i.e. a bijective function.)

A function f that is not injective is sometimes called many-to-one. However, this name too is best avoided, since it is sometimes used to mean "single-valued" — i.e. each argument is mapped to at most one value.

Contents 1 Examples and counter-examples 2 Injections are invertible 3 Other properties 4 Category theory view 5 See also

[edit] Examples and counter-examples

• For any set X, the identity function on X is injective.
• The function f : R → R defined by f(x) = 2x + 1 is injective.
• The function g : R → R defined by g(x) = x² is not injective, because (for example) g(1) = 1 = g(−1). However, if g is redefined so that its domain is the non-negative real numbers [0,+∞), then g is injective.
• The exponential function is injective.
• The natural logarithm function is injective.
• The function g : R → R defined by g(x) = x³ − x is not injective, since, for example, g(0) = g(1).

More generally, when X and Y are both the real line R, then an injective function f : R → R is one whose graph is never intersected by any horizontal line more than once.

[edit] Injections are invertible

Another definition of injection is a function whose effect can be undone. More precisely, f : X → Y is injective if there exists a function g : Y → X such that g(f(x)) = x for every x in ´ X; that is, g o f  equals the identity function on X.

Note that g may not be a complete inverse of f because the composition in the other order, f o g, may not be the identity on Y.

In fact, to turn an injective function f : X → Y into a bijective (hence invertible) function, it suffices to replace its codomain Y by its actual range J = f(X). That is, let g : X → J such that g(x) = f(x) for all x in X; then g is bijective. Indeed, f can be factored as incl_J,Yog, where incl_J,Yis the inclusion function from J into Y.

[edit] Other properties

• If f and g are both injective, then f o g is injective.

• If g o f is injective, then f is injective (but g need not be).
• f : X → Y is injective if and only if, given any functions g, h : W → X, whenever f o g = f o h, then g = h.
• If f : X → Y is injective and A is a subset of X, then f ⁻¹(f(A)) = A. Thus, A can be recovered from its image f(A).
• If f : X → Y is injective and A and B are both subsets of X, then f(A ∩ B) = f(A) ∩ f(B).
• Every function h : W → Y can be decomposed as h = f o g for a suitable injection f and surjection g. This decomposition is unique up to isomorphism, and f may be thought of as the inclusion function of the range h(W) of h as a subset of the codomain Y of h.
• If f : X → Y is an injective function, then Y has at least as many elements as X, in the sense of cardinal numbers.
• If both X and Y are finite with the same number of elements, then f : X → Y is injective if and only if f is surjective.
• Every embedding is injective.

[edit] Category theory view

In the language of category theory, injective functions are precisely the monomorphisms in the category of sets.

[edit] See also

• bijective function
• injective module
• monomorphism
• surjective function
• Horizontal line test
• Injective metric space