# Inverse of Strictly Monotone Function

Let $f:\mathbb{R}\to\mathrm{S}$, where $\mathrm{S}\subseteq\mathbb{R}$, be a strictly increasing (decreasing) function. I wish to find out whether its inverse $f^{-1}$ is also strictly increasing (decreasing) or not. While a proof for this exists online, I will provide a counterexample that seems to contradict the conclusions in the proof.

The proof can be found in: https://proofwiki.org/wiki/Inverse_of_Strictly_Monotone_Function

I will for simplicity only consider a strictly increasing function. From the provided link above we have that: \begin{align*} x < y & \quad\Leftrightarrow\quad f(x) < f(y) \\ f^{-1}(x) < f^{-1}(y) & \quad\Leftrightarrow\quad f^{-1}(f(x)) < f^{-1}(f(y)) \\ f^{-1}(x) < f^{-1}(y) & \quad\Leftrightarrow\quad x < y \\ \end{align*} Thus, if $f$ is a strictly increasing function, then so is $f^{-1}$.

Now for the counterexample. With some loss of generality, let $f:\mathbb{R}\to\mathbb{R}_{++}$ be a strictly positive function, where $\mathbb{R}_{++}=\{ x\in\mathbb{R}: x>0\}$. Furthermore, for an inverse it follows that $f^{-1}(x)f(x)=f(x)f^{-1}(x)=1$. Then from \begin{align*} x < y & \quad\Leftrightarrow\quad f(x) < f(y) \end{align*} we can rewrite the right-hand side as \begin{align*} f^{-1}(y)=\frac{1}{f(y)}<\frac{1}{f(x)}=f^{-1}(x) \end{align*} which implies that if $f$ is a strictly increasing function, then its inverse $f^{-1}$ is strictly decreasing.

• The reciprocal is different from the function inverse! A way to see the theorem heuristically is that the inverse is in some sense a reflection over the line $y=x$, and from there you can see the result with some doodling. – Andres Mejia Nov 2 '16 at 14:20
• Yes, I seem to have confused inverse with reciprocal. In the proof, how is one allowed to take $f^{−1}(\cdot)$ over $x<y$ on both sides without knowing if the "less than" sign should change direction? Doesn't this imply already that $f^{−1}$ is assumed to be strictly increasing? – index Nov 2 '16 at 14:36

We may argue as follows:

Let $$f:X \mapsto Y$$ be a monotone increasing function ( hence injective ), and let $$y_1, y_2 \in Y$$, such that $$y_1 < y_2$$. Then, $$\exists \ x_1, x_2 \in X \$$ s.t. $$x_1=f^{-1}(y_1), x_2=f^{-1}(y_2)$$. Thus, we have $$f(x_1)=y_1, f(x_2)=y_2$$.

Now, we claim that $$f^{-1}(y_1) < f^{-1}(y_2)$$. Otherwise, $$f^{-1}(y_1) \geq f^{-1}(y_2) \Rightarrow x_1 \geq x_2 \Rightarrow f(x_1) \geq f(x_2) \Rightarrow y_1 \geq y_2$$, which is obviously a contradiction, so this concludes our proof.

It appears that you may be confusing the reciprocal function $\frac{1}{f(x)}$ with the inverse function (denoted $f^{-1}(x)$).

This latter notation, $f^{-1}$, is usually reserved for, and understood to be, the inverse function rather than the reciprocal function.

Edit:

In response to index's comment: I would prefer to structure the proof as:

We know $x<y \Leftrightarrow f(x)<f(y)$.

Hence $f^{-1}(x)<f^{-1}(y) \Leftrightarrow f(f^{-1}(x))<f(f^{-1}(y)) \Leftrightarrow x<y$.

• Yes, I seem to have confused inverse with reciprocal. In the proof, how is one allowed to take $f^{-1}(\cdot)$ over $x<y$ on both sides without knowing if the "less than" sign should change direction? Doesn't this imply already that $f^{-1}$ is assumed to be strictly increasing? – index Nov 2 '16 at 14:08
• Could you please comment a bit more on your edited part and your thinking? I don't see how your restructure of the proof answers my comment? – index Nov 3 '16 at 10:34