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Injective composition: the second function need not be injective. A function is injective (one-to-one) if each possible element of the codomain is mapped to by at most one argument. Equivalently, a function is injective if it maps distinct arguments to distinct images. An injective function is an injection. [1] The formal definition is the ...
Functions which satisfy property (4) are said to be "one-to-one functions" and are called injections (or injective functions). [2] With this terminology, a bijection is a function which is both a surjection and an injection, or using other words, a bijection is a function which is both "one-to-one" and "onto". [3]
Bijective function: is both an injection and a surjection, and thus invertible. Identity function: maps any given element to itself. Constant function: has a fixed value regardless of its input. Empty function: whose domain equals the empty set. Set function: whose input is a set.
In other words, the function value f(x) in the codomain X is always the same as the input element x in the domain X.The identity function on X is clearly an injective function as well as a surjective function (its codomain is also its range), so it is bijective.
Variations of the horizontal line test can be used to determine whether a function is surjective or bijective: The function f is surjective (i.e., onto) if and only if its graph intersects any horizontal line at least once. f is bijective if and only if any horizontal line will intersect the graph exactly once.
The composition of one-to-one (injective) functions is always one-to-one. Similarly, the composition of onto (surjective) functions is always onto. It follows that the composition of two bijections is also a bijection. The inverse function of a composition (assumed invertible) has the property that (f ∘ g) −1 = g −1 ∘ f −1.
This example is injective in each input separately, because the functions f x and f y are always injective. However, it's not injective in both variables simultaneously, because (for example) f (2,4) = f (1,2). One can also consider partial binary functions, which may be defined only for certain values of the inputs.
Mathematically, a bidirectional map can be defined a bijection: between two different sets of keys and of equal cardinality, thus constituting an injective and surjective function: { ∀ x , x ′ ∈ X , f ( x ) = f ( x ′ ) ⇒ x = x ′ ∀ y ∈ Y , ∃ x ∈ X : y = f ( x ) ⇒ ∃ f − 1 ( x ) {\displaystyle {\begin{cases}&\forall x,x ...