# Definition:Inverse Sine/Real/Arcsine

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## 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 **arcsine of $x$** and is written $\arcsin x$.

Thus:

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

## Also denoted as

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

## Caution

There exists the a 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 an 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 $\arcsin$, although this can also be confusing due to the visual similarity between that and the lowercase $\text s$.

## 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 - 2014: Christopher Clapham and James Nicholson:
*The Concise Oxford Dictionary of Mathematics*(5th ed.) ... (previous) ... (next): Entry:**inverse trigonometric function**