Devil’s Staircase

The ‘Devils Staircase’ is a famous function in mathematics. It is a non-constant function that has $0$ derivative almost everywhere. Further it is continuous but not absolutely continuous. Constructing the devils staircase is easy, and is done recursively. We start with the function $f_{0}:[0,1]\to[0,1]$ where $f_{0}(x)=x$. Now suppose we have defined $f_{n}$ we define $f_{n+1}$ by letting it be $1/2$ on the interval $[1/3,2/3]$, then we take a copy of the full function $f_{n}$ and “squash” it into the box with vertices : $(0,0),(1/3,0),(1/3,1/2),(0,1/2)$, then again we take a copy of $f_{n}$ and we “squash” it into the box with...

Incorrect usage of Induction

Induction is often incorrectly used by beginning students by ‘extending it to infinity’ . For example suppose the problem is the following: Problem: Prove/Disprove that the product of $\mathbb{N}$ number of countable sets is countable. Often many students show this kind of reasoning: since the product of two sets is countable, and the product of three sets is countable and if we know that the product of $n$ sets is countable then so is the product of $n+1$ sets, therefore we can conclude by induction that the product $\mathbb{N}$ number of countable sets is countable. The problem with this kind...