# Definition:Inverse Cotangent/Real/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:

## 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.**

## Also denoted as

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

- $\arccot$
- $\operatorname {acot}$
- $\operatorname {actn}$

## Also see

- Results about
**inverse cotangent**can be found here.

### Other inverse trigonometrical ratios

- Definition:Arcsine
- Definition:Arccosine
- Definition:Arctangent
- Definition:Arcsecant
- Definition:Arccosecant

## Sources

- 1968: Murray R. Spiegel:
*Mathematical Handbook of Formulas and Tables*... (previous) ... (next): $\S 5$: Trigonometric Functions: Principal Values for Inverse Trigonometrical Functions