Dear community, I am having some trouble with a probably a stupid problem, I looked carefully in the documentation but I did not be able to find a solution. The problem is very easy to explain, if I define a function, for example:

f[x]:=a x.

Now, if I try using the Manipulate function in this way:

Manipulate[Plot[f[x],{x,0,3}],{a,0,1}],

the program shows me the Manipulate's box, but without the drawing of the function! But if I type:

 Manipulate[Plot[a x,{x,0,3}],{a,0,1}]

it works fine. Obviously either the Plot function and the line that I used are just an example, Mathematica does not work with any function. So, do I am making some error with the Mathematica syntax or does not Manipulate support the user defined functions? Thanks you for the answer