Functions:Injective

In mathematics, an injective function (also known as injection, or one-to-one function) is a function f that maps distinct elements to distinct elements; that is, f(x₁) = f(x₂) implies x₁ = x₂. In other words, every element of the function's codomain is the image of at most one element of its domain. 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.

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  An injective non-surjective function (injection, not a bijection)

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  An injective surjective function (bijection)

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  A non-injective surjective function (surjection, not a bijection)

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  A non-injective non-surjective function (also not a bijection)

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. 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.

Definition

Let $f$ be a function whose domain is a set $X.$ The function $f$ is said to be injective provided that for all $a$ and $b$ in $X,$ if $f(a)=f(b),$ then $a=b$; that is, $f(a)=f(b)$ implies $a=b.$ Equivalently, if $a\neq b,$ then $f(a)\neq f(b).$

Symbolically,

$$\forall a,b\in X,\;\;f(a)=f(b)\Rightarrow a=b,$$

which is logically equivalent to the contrapositive,

$$\forall a,b\in X,\;\;a\neq b\Rightarrow f(a)\neq f(b).$$

Examples

Injective functions. Diagramatic interpretation in the Cartesian plane, defined by the mapping $f:X\to Y,$ where $y=f(x),$ $X=$ domain of function, $Y=$ range of function, and $\operatorname {im} (f)$ denotes image of $f.$ Every one $x$ in $X$ maps to exactly one unique $y$ in $Y.$ The circled parts of the axes represent domain and range sets— in accordance with the standard diagrams above.

• For any set $X$ and any subset $S\subseteq X,$ the inclusion map $S\to X$ (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).
• If the domain of a function is the empty set, then the function is the empty function, which is injective.
• If the domain of a function has one element (that is, it is a singleton set), then the function is always injective.
• 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 exponential function $\exp :\mathbb {R} \to \mathbb {R}$ defined by $\exp(x)=e^{x}$ is injective (but not surjective, as no real value maps to a negative number).
• The natural logarithm function $\ln :(0,\infty )\to \mathbb {R}$ defined by $x\mapsto \ln x$ is injective.
• The function $g:\mathbb {R} \to \mathbb {R}$ defined by $g(x)=x^{n}-x$ is not injective, since, for example, $g(0)=g(1)=0.$

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.

Not an injective function. Here $X_{1}$ and $X_{2}$ are subsets of $X,Y_{1}$ and $Y_{2}$ are subsets of $Y$: for two regions where the function is not injective because more than one domain element can map to a single range element. That is, it is possible for more than one $x$ in $X$ to map to the same $y$ in $Y.$

Making functions injective. The previous function $f:X\to Y$ can be reduced to one or more injective functions (say) $f:X_{1}\to Y_{1}$ and $f:X_{2}\to Y_{2},$ shown by solid curves (long-dash parts of initial curve are not mapped to anymore). Notice how the rule $f$ has not changed – only the domain and range. $X_{1}$ and $X_{2}$ are subsets of $X,Y_{1}$ and $Y_{2}$ are subsets of $Y$: for two regions where the initial function can be made injective so that one domain element can map to a single range element. That is, only one $x$ in $X$ maps to one $y$ in $Y.$

Injections can be undone

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 $x\in X,$

$g(f(x))=x$ ($f$ can be undone by $g$), 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 non-empty domain has a left inverse $g,$ which can be defined by fixing an element $a$ in the domain of $f$ so that $g(x)$ equals the unique pre-image of $x$ under $f$ if it exists and $g(x)=1$ otherwise.

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

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 range $J=f(X).$ 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.

Other properties

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

The composition of two injective functions 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 monomorphisms in the category Set of sets.
• If $f:X\to Y$ 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 $f:X\to Y$ 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 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 $f:X\to Y$ is an injective function, then $Y$ has at least as many elements as $X,$ in the sense of cardinal numbers. 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.)
• If both $X$ and $Y$ are finite 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).
• 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

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 $x=y.$

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.

Resources

• Function Types, OpenStax
• Course Textbook, pages 154-164

Licensing

Content obtained and/or adapted from:

• Injective Function, Wikipedia under a CC BY-SA license