## Increasing functions and functions increasing

• mathematics • teaching • KS5 • PermalinkHere’s the graph of $y=-\dfrac{1}{x}$ for $x\ne0$.

Where is this function increasing? Is it an increasing function?

Looking at various recent examination papers, it has become clear to me that there is significant confusion between these two questions. This post is intended to bring some clarity to the situation.

At the start of this post, I will give an example of the confusion as it appears in exam questions (and probably elsewhere), and clarify what the two different phrases mean using the above example. I will then delve more deeply into the mathematics of these two things, going beyond A-level content, and use some undergraduate analysis to find equivalent conditions for them in terms of the derivatives of the functions. It is fine to skip over the technical stuff and just look at the results (theorems)!

(Exactly the same applies to the use of the term “decreasing”, but for simplicity we will focus on increasing functions in this post.)