Set Theory – Cardinality of Set of Everywhere-Discontinuous Functions

Question

The question is to find the cardinality of the set of all everywhere-discontinous real-valued functions of real variable.

My intuition tells me there are $2^c$ such functions, but I can't seem to find an injection from the set of all functions to the set of everywhere-discontinuous functions.

Any help would be appreciated.

$c$ here denotes the cardinality of continuum (for an example, the cardinality of set of all real numbers).

Answer 1

Your intuition is correct. Here's one way to prove it:

Write $\mathbb{Q}$ as the disjoint union of two dense sets $A, B$ (e.g. take $A$ to be the dyadic rationals and $B=\mathbb{Q}\setminus A$). Then:

Any function $f$ satisfying $f(a)=1$ for $a\in A$, $f(b)=0$ for $b\in B$ is everywhere discontinuous.

So how many functions of this type are there? Well, there's no restriction on the behavior of $f$ on irrational inputs, so we have:

The number of everywhere discontinuous functions is at least the number of functions from the irrationals to the reals.

Now using the fact that the irrationals have cardinality $c$, do you see how to finish the proof?