Definition:Inverse Secant/Real

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Definition

Let $x \in \R$ be a real number such that $x \le -1$ or $x \ge 1$.

The inverse secant of $x$ is the multifunction defined as:

$\map {\sec^{-1} } x := \set {y \in \R: \map \sec y = x}$

where $\map \sec y$ is the secant of $y$.


Arcsecant

Arcsecant Function

From Shape of Secant Function, we have that $\sec x$ is continuous and strictly increasing on the intervals $\hointr 0 {\dfrac \pi 2}$ and $\hointl {\dfrac \pi 2} \pi$.

From the same source, we also have that:

$\sec x \to + \infty$ as $x \to \dfrac \pi 2^-$
$\sec x \to - \infty$ as $x \to \dfrac \pi 2^+$


Let $g: \hointr 0 {\dfrac \pi 2} \to \hointr 1 \to$ be the restriction of $\sec x$ to $\hointr 0 {\dfrac \pi 2}$.

Let $h: \hointl {\dfrac \pi 2} \pi \to \hointl \gets {-1}$ be the restriction of $\sec x$ to $\hointl {\dfrac \pi 2} \pi$.

Let $f: \closedint 0 \pi \setminus \dfrac \pi 2 \to \R \setminus \openint {-1} 1$:

$\map f x = \begin{cases} \map g x & : 0 \le x < \dfrac \pi 2 \\ \map h x & : \dfrac \pi 2 < x \le \pi \end{cases}$


From Inverse of Strictly Monotone Function, $\map g x$ admits an inverse function, which will be continuous and strictly increasing on $\hointr 1 \to$.

From Inverse of Strictly Monotone Function, $\map h x$ admits an inverse function, which will be continuous and strictly increasing on $\hointl \gets {-1}$.

As both the domain and range of $g$ and $h$ are disjoint, it follows that:

$\map {f^{-1} } x = \begin{cases} \map {g^{-1} } x & : x \ge 1 \\ \map {h^{-1} } x & : x \le -1 \end{cases}$


This function $f^{-1} \left({x}\right)$ is called arcsecant of $x$ and is written $\arcsec x$.

Thus:

The domain of $\arcsec x$ is $\R \setminus \openint {-1} 1$
The image of $\arcsec x$ is $\closedint 0 \pi \setminus \dfrac \pi 2$.