# Definition:Inverse Cotangent/Real/Arccotangent

(Redirected from Definition:Arccotangent)

## Definition

From Shape of Cotangent Function, we have that $\cot x$ is continuous and strictly decreasing on the interval $\openint 0 \pi$.

From the same source, we also have that:

$\cot x \to + \infty$ as $x \to 0^+$
$\cot x \to - \infty$ as $x \to \pi^-$

Let $g: \openint 0 \pi \to \R$ be the restriction of $\cot x$ to $\openint 0 \pi$.

Thus from Inverse of Strictly Monotone Function, $\map g x$ admits an inverse function, which will be continuous and strictly decreasing on $\R$.

This function is called arccotangent of $x$ and is written $\arccot x$.

Thus:

The domain of the arccotangent is $\R$
The image of the arccotangent is $\openint 0 \pi$.

## Terminology

There exists the popular but misleading notation $\cot^{-1} x$, which is supposed to denote the inverse cotangent function.

However, note that as $\cot x$ is not an injection, it does not have a well-defined inverse.

The $\arccot$ function as defined here has a well-specified image which (to a certain extent) is arbitrarily chosen for convenience.

Therefore it is preferred to the notation $\cot^{-1} x$, which (as pointed out) can be confusing and misleading.

Sometimes, $\operatorname {Cot}^{-1}$ (with a capital $\text C$) is taken to mean the same as $\arccot$.

However this can also be confusing due to the visual similarity between that and the lowercase $\text c$.

Some sources hyphenate: arc-cotangent.

## Symbol

The symbol used to denote the arccotangent function is variously seen as follows:

arccot

The usual symbol used on $\mathsf{Pr} \infty \mathsf{fWiki}$ to denote the arccotangent function is $\arccot$.

acot

A variant symbol used to denote the arccotangent function is $\operatorname {acot}$.

actn

A variant symbol used to denote the arccotangent function is $\operatorname {actn}$.

## Also see

• Results about inverse cotangent can be found here.