MATH1231 - Functions

# Injective / One-to-One

A function is injective if no point in the codomain of the function is the function value of more than one point in the domain.

Essentially each y corresponds to only one x.

(1)\begin{align} f : X \rightarrow Y \text { is injective if } f(x_1) = f(x_2) \text { only when } x_1 = x_2 \end{align}

# Surjective / Onto

A function is surjective if every point in the range of the function is in the range. I.e. range = codomain.

Essentially every y exists.

(2)\begin{align} f : X \rightarrow Y \text { is surjective if } \forall y \in Y \exists x \in X \text { such that } y = f(x) \end{align}

# Bijective

A function is bijective if it is both injective and surjective.

Essentially every x has one y, and every y has an x.

# Inverse

A function has an inverse iff the function is bijective.

A function **is** an inverse if:

- g o f = idX (i.e. idX(x) = x)
- f o g = idY (i.e. idY(y) = y)

page revision: 4, last edited: 05 Sep 2011 12:03