I'd like to construct a function Q that has a symbolic expression f(u1,u2,...,ui) as an argument.
The thing is that I don't want to declare f as being an expression of symbolic variables u1,...,ui.
I want Q to do this for me: it receives some expression with u1,...,ui (without these being defined anything) and then it declares these all as symbolic.
Is this possible?
No, in order to do anything close to that, you would have to pass the expression as a string (or as a function handle, or as an inline function)