How to make clear a letter is a function?

Question

How should I make clear that a symbol is a function?

Usually a function is denoted by the letter $f$ or $g$, or is directly applied to arguments (e.g. $c(x,y)$) or is implied to be a function by an operation on functions (e.g. $h \circ j$), but in my case renaming the function would be confusing and there is not a lot of context making clear that it's a function.

Could I annotate the letter in some way which makes clear that it's a function? I am thinking of something like $\widetilde{c}$ or some other symbol above the letter.

Is there some notation by which I can make clear that $c$ is a function?

Answer 1

The easiest and most direct way to say that $f$ is a function is by stating it as such. Actually, even if you use the letter $f$, you should always explicitly state that $f$ denotes a function. For example, this statement:

If $f$ is a differentiable function, then $f$ is continuous

is, by my oppinion, much better than

If $f$ is differentiable, then $f$ is continuous.

If you are always careful to tell what type of variable each letter you introduce represents, then using "correct" symbols ($f$, $g$ for functions, $c$ for constants) is not strictly necesary.

Answer 2

Another idea is to indicate the domain and codomain. This makes it clear the object you're considering is a function.

Suppose $f \colon (0,1) \to \mathbb R$ is differentiable. Then $f$ is continuous.