Difference between function and expression

### 2.15.1 Difference between function and expression

A function `f` is defined for example by :
`f(x):=x```^```2-1` or by `f:=x->x```^```2-1`
that is to say, for all x, f(x) is equal to the expression x2−1. In that case, to have the value of f for x=2, input :`f(2)`.
But if the input is `g:=x```^```2-1`, then `g` is a variable where the expression x2−1 is stored. In that case, to have the value of g for x=2, input : `subst(g,x=2)` (g is an expression depending on x).

When a command expects a function as argument, this argument should be either the definition of the function (e.g. `x->x```^```2-1`) or a variable name assigned to a function (e.g. `f` previously defined by e.g. `f(x):=x```^```2-1`).
When a command expects an expression as argument, this argument should be either the definition of the expression (for example `x```^```2-1`), or a variable name assigned to an expression (e.g. `g` previously defined, for example, by `g:=x```^```2-1`), or the evaluation of a function. e.g. `f(x)` if `f` is a previously defined function, for example, by `f(x):=x```^```2-1`).