What is a basis for the vector space of continuous functions?
Solution 1:
There is, in a fairly strong sense, no reasonable basis of this space. Zoom in on a neighborhood at any point and note that a finite linear combination of functions which have various kinds of nice behavior in that neighborhood also has that nice behavior in that neighborhood (differentiable, $C^k$, smooth, etc.). So any basis necessarily contains, for every such neighborhood, a function which does not behave nicely in that neighborhood. More generally, but roughly speaking, a basis needs to have functions which are at least as pathological as the most pathological continuous functions.
(Hamel / algebraic) bases of most infinite-dimensional vector spaces simply are not useful. In applications, the various topologies you could put on such a thing matter a lot and the notion of a Schauder basis becomes more useful.
Solution 2:
Using Nate Eldredge's comment we have that $C(\mathbb R)$ is a Polish vector space.
Consider a Solovay model, that is ZF+DC+"All sets have the Baire property". In such model all linear maps into separable vector spaces are continuous, this is a consequence of [1, Th. 9.10].
It is important to remark that a continuous function (from $\mathbb R$ to $\mathbb R$) from a compact set is uniformly continuous is a result which do not require any form of choice, and I believe that Dependent Choice (DC) ensures that uniform converges on compact sets is well behaved.
Suppose that there was a Hamel basis, $B$, it has to be of cardinality $\frak c$. So it has $2^\frak c$ many permutations, which induce $2^\frak c$ different linear automorphisms.
However every linear automorphism is automatically continuous, so it is determined completely by the countable dense set, and therefore there can only be $\frak c$ many linear automorphisms which is a contradiction to Cantor's theorem since $\mathfrak c\neq 2^\frak c$.
This is essentially the same argument as I used in this answer.
Bibliography:
- Kechris, A. Classical Descriptive Set Theory. Springer-Verlag, 1994.
Solution 3:
I'll use faith to believe we are in one of those situations described by the axiom of choice ; had one discovered a useful basis for this vector space, it'd be known all over the place. The best we have as a basis right now, (and the word "best" means 'to my belief, the one that looks the prettiest') is the fact that the functions $e^{inx}$ with $n \in \mathbb Z$ form a Schauder basis of the Hilbert space $L^2([a,b])$ of $\mathbb C$-valued functions (modulo functions which are zero almost everywhere on $[a,b]$ ; note that this is not a Hamel basis). For real valued functions, take the functions $\sin(nx)$ and $\cos(nx)$ as your basis.
Hope that helps,