Injective function

In mathematics, an injective function (also known as injection, or one-to-one function^[1] ) is a function $f$ that maps distinct elements of its domain to distinct elements; that is, $x 1 \neq x 2$ implies $f (x 1) \neq f (x 2)$ . (Equivalently, $f (x 1) = f (x 2)$ implies $x 1 = x 2$ in the equivalent contrapositive statement.) In other words, every element of the function's codomain is the image of at most one element of its domain.^[2] The term one-to-one function must not be confused with one-to-one correspondence that refers to bijective functions, which are functions such that each element in the codomain is an image of exactly one element in the domain.

"Injective" redirects here. For other uses, see Injective module and Injective object.

A homomorphism between algebraic structures is a function that is compatible with the operations of the structures. For all common algebraic structures, and, in particular for vector spaces, an injective homomorphism is also called a monomorphism. However, in the more general context of category theory, the definition of a monomorphism differs from that of an injective homomorphism.^[3] This is thus a theorem that they are equivalent for algebraic structures; see Homomorphism § Monomorphism for more details.

A function $f$ that is not injective is sometimes called many-to-one.^[2]

For any set $X$ and any subset $S\subseteq X,$ the $S\to X$ (which sends any element $s\in S$ to itself) is injective. In particular, the identity function $X\to X$ is always injective (and in fact bijective).

inclusion map

If the domain of a function is the , then the function is the empty function, which is injective.

empty set

If the domain of a function has one element (that is, it is a ), then the function is always injective.

singleton set

The function $f:\mathbb {R} \to \mathbb {R}$ defined by $f(x)=2x+1$ is injective.

The function $g:\mathbb {R} \to \mathbb {R}$ defined by $g(x)=x^{2}$ 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 $\exp :\mathbb {R} \to \mathbb {R}$ defined by $\exp(x)=e^{x}$ is injective (but not surjective, as no real value maps to a negative number).

exponential function

The function $\ln :(0,\infty )\to \mathbb {R}$ defined by $x\mapsto \ln x$ is injective.

natural logarithm

The function $g:\mathbb {R} \to \mathbb {R}$ defined by $g(x)=x^{n}-x$ is not injective, since, for example, $g(0)=g(1)=0.$

For visual examples, readers are directed to the gallery section.

More generally, when $X$ and $Y$ are both the real line $\mathbb {R} ,$ then an injective function $f:\mathbb {R} \to \mathbb {R}$ is one whose graph is never intersected by any horizontal line more than once. This principle is referred to as the horizontal line test.^[2]

Injections can be undone[edit]

Functions with left inverses are always injections. That is, given $f:X\to Y,$ if there is a function $g:Y\to X$ such that for every $x\in X$ , $g(f(x))=x$ , then $f$ is injective. In this case, $g$ is called a retraction of $f.$ Conversely, $f$ is called a section of $g.$

Conversely, every injection $f$ with a non-empty domain has a left inverse $g$ . It can be defined by choosing an element $a$ in the domain of $f$ and setting $g(y)$ to the unique element of the pre-image $f^{-1}[y]$ (if it is non-empty) or to $a$ (otherwise).^[5]

The left inverse $g$ is not necessarily an inverse of $f,$ because the composition in the other order, $f\circ g,$ may differ from the identity on $Y.$ In other words, an injective function can be "reversed" by a left inverse, but is not necessarily invertible, which requires that the function is bijective.

Injections may be made invertible[edit]

In fact, to turn an injective function $f:X\to Y$ into a bijective (hence invertible) function, it suffices to replace its codomain $Y$ by its actual image $J=f(X).$ That is, let $g:X\to J$ such that $g(x)=f(x)$ for all $x\in X$ ; then $g$ is bijective. Indeed, $f$ can be factored as $\operatorname {In} _{J,Y}\circ g,$ where $\operatorname {In} _{J,Y}$ is the inclusion function from $J$ into $Y.$

More generally, injective partial functions are called partial bijections.

If $f$ and $g$ are both injective then $f\circ g$ is injective.

If $g\circ f$ is injective, then $f$ is injective (but $g$ need not be).

$f:X\to Y$ is injective if and only if, given any functions $g,$ $h:W\to X$ whenever $f\circ g=f\circ h,$ then $g=h.$ In other words, injective functions are precisely the in the category Set of sets.

monomorphisms

If $f:X\to Y$ is injective and $A$ is a of $X,$ then $f^{-1}(f(A))=A.$ Thus, $A$ can be recovered from its image $f(A).$

subset

If $f:X\to Y$ is injective and $A$ and $B$ are both subsets of $X,$ then $f(A\cap B)=f(A)\cap f(B).$

Every function $h:W\to Y$ can be decomposed as $h=f\circ g$ for a suitable injection $f$ and surjection $g.$ This decomposition is unique , 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 $h.$

up to isomorphism

If $f:X\to Y$ is an injective function, then $Y$ has at least as many elements as $X,$ in the sense of . In particular, if, in addition, there is an injection from $Y$ to $X,$ then $X$ and $Y$ have the same cardinal number. (This is known as the Cantor–Bernstein–Schroeder theorem.)

cardinal numbers

If both $X$ and $Y$ are with the same number of elements, then $f:X\to Y$ is injective if and only if $f$ is surjective (in which case $f$ is bijective).

finite

An injective function which is a homomorphism between two algebraic structures is an .

embedding

Unlike surjectivity, which is a relation between the graph of a function and its codomain, injectivity is a property of the graph of the function alone; that is, whether a function $f$ is injective can be decided by only considering the graph (and not the codomain) of $f.$

Proving that functions are injective[edit]

A proof that a function $f$ is injective depends on how the function is presented and what properties the function holds. For functions that are given by some formula there is a basic idea. We use the definition of injectivity, namely that if $f(x)=f(y),$ then $x=y.$ ^[6]

Here is an example: $f(x)=2x+3$

Proof: Let $f:X\to Y.$ Suppose $f(x)=f(y).$ So $2x+3=2y+3$ implies $2x=2y,$ which implies $x=y.$ Therefore, it follows from the definition that $f$ is injective.

There are multiple other methods of proving that a function is injective. For example, in calculus if $f$ is a differentiable function defined on some interval, then it is sufficient to show that the derivative is always positive or always negative on that interval. In linear algebra, if $f$ is a linear transformation it is sufficient to show that the kernel of $f$ contains only the zero vector. If $f$ is a function with finite domain it is sufficient to look through the list of images of each domain element and check that no image occurs twice on the list.

A graphical approach for a real-valued function $f$ of a real variable $x$ is the horizontal line test. If every horizontal line intersects the curve of $f(x)$ in at most one point, then $f$ is injective or one-to-one.