# Definition:Inverse Sine/Real/Arcsine

Although this article appears correct, it's inelegant. There has to be a better way of doing it.In particular: Rewrite this in less discursive termsYou can help $\mathsf{Pr} \infty \mathsf{fWiki}$ by redesigning it.To discuss this page in more detail, feel free to use the talk page.When this work has been completed, you may remove this instance of `{{Improve}}` from the code.If you would welcome a second opinion as to whether your work is correct, add a call to `{{Proofread}}` the page. |

## Definition

From Shape of Sine Function, we have that $\sin x$ is continuous and strictly increasing on the interval $\closedint {-\dfrac \pi 2} {\dfrac \pi 2}$.

From Sine of Half-Integer Multiple of Pi:

- $\map \sin {-\dfrac {\pi} 2} = -1$

and:

- $\sin \dfrac {\pi} 2 = 1$

Therefore, let $g: \closedint {-\dfrac \pi 2} {\dfrac \pi 2} \to \closedint {-1} 1$ be the restriction of $\sin x$ to $\closedint {-\dfrac \pi 2} {\dfrac \pi 2}$.

Thus from Inverse of Strictly Monotone Function, $g \paren x$ admits an inverse function, which will be continuous and strictly increasing on $\closedint {-1} 1$.

This function is called the **arcsine of $x$**.

Thus:

- The domain of
**arcsine**is $\closedint {-1} 1$ - The image of
**arcsine**is $\closedint {-\dfrac \pi 2} {\dfrac \pi 2}$.

## Symbol

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

The usual symbol used on $\mathsf{Pr} \infty \mathsf{fWiki}$ to denote the **arcsine function** is $\arcsin$.

A variant symbol used to denote the **arcsine function** is $\operatorname {asin}$.

## Terminology

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

However, note that as $\sin x$ is not an injection (even though by restriction of the codomain it can be considered surjective), it does not have a well-defined inverse.

The $\arcsin$ 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 $\sin^{-1} x$, which (as pointed out) can be confusing and misleading.

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

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

In computer software packages, the notation $\operatorname {asin}$ or $\operatorname {asn}$ can sometimes be found.

Some sources hyphenate: **arc-sine.**

## Also see

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

### Other inverse trigonometrical ratios

- Definition:Arccosine
- Definition:Arctangent
- Definition:Arccotangent
- 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 - 1977: K.G. Binmore:
*Mathematical Analysis: A Straightforward Approach*... (previous) ... (next): $\S 16.5 \ (3)$ - 1978: Thomas A. Whitelaw:
*An Introduction to Abstract Algebra*... (previous) ... (next): $\S 23$: Restriction of a Mapping - 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next):**inverse trigonometric functions (antitrigonometric functions)** - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**inverse trigonometric functions (antitrigonometric functions)** - 2014: Christopher Clapham and James Nicholson:
*The Concise Oxford Dictionary of Mathematics*(5th ed.) ... (previous) ... (next):**inverse trigonometric function**