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For any set X, the identity function 1 X: X → X, 1 X (x) = x is bijective. The function f: R → R, f(x) = 2x + 1 is bijective, since for each y there is a unique x = (y − 1)/2 such that f(x) = y. More generally, any linear function over the reals, f: R → R, f(x) = ax + b (where a is non-zero) is a bijection.
Moreover, f is the composition of the canonical projection from f to the quotient set, and the bijection between the quotient set and the codomain of . The composition of two surjections is again a surjection, but if g ∘ f {\displaystyle g\circ f} is surjective, then it can only be concluded that g {\displaystyle g} is surjective (see figure).
The function f : R → R defined by f(x) = x 3 − 3x is surjective, because the pre-image of any real number y is the solution set of the cubic polynomial equation x 3 − 3x − y = 0, and every cubic polynomial with real coefficients has at least one real root.
Given a function :, the canonical surjection of f onto its image () = {()} is the function from X to f(X) that maps x to f(x). For every subset A of a set X, the inclusion map of A into X is the injective (see below) function that maps every element of A to itself.
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 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. [2] The identity function f on X is often denoted by id X. In set theory, where a function is defined as a particular kind of binary relation, the identity function is given by the identity relation ...
Formally, for each x: f (x) = f (−x). Odd function: is symmetric with respect to the origin. Formally, for each x: f (−x) = −f (x). Relative to a binary operation and an order: Subadditive function: for which the value of f (x + y) is less than or equal to f (x) + f (y).
A faithful functor need not be injective on objects or morphisms. That is, two objects X and X′ may map to the same object in D (which is why the range of a full and faithful functor is not necessarily isomorphic to C), and two morphisms f : X → Y and f′ : X′ → Y′ (with different domains/codomains) may map to the same morphism in D.