# Difference Between Constants and Variables

Hello all. I'm working with variational calculus in sage and working with
complicate expression `F`

of a lot of variables declared with `var`

.
I used `var`

to declare both variables and constants. My problem is that I have
to know the exact number of variables which some generic expression depends. By
this reason the member function `variables`

of an expression does not work, i.e.,
it returs both variables and constants because them was declared with `var`

function.
I'm wondering if there is some type of variable in sage that can act as a variable to construct an expression and don't be returned by the `variables`

member function.

Could you please give an example of your expression, and tell us what you consider as variable and what you consider as constant in your example ?