Not surprised that `f(3)`

is giving you an error. This form of substituting values in symbolic expressions has been deprecated for quite a while (well, over four years!). You must have got the warning at some point in your calculations. You should be using either `f.subs(x=3)`

or `f(x=3)`

, i.e., specify the variable name explicitly.

Note that there are some idiosyncrasies while working with symbolic expressions. For instance the following will not complain even though `f`

is not defined as a function of `x`

:

```
sage: f = SR(1)
sage: f.subs(x=2)
1
```