When a function has a rather complex set of parameters, the function identifier (usually simply f), gets dwarfed by the parentheses and the parameters inside of them:
As you can see in the red circle, the "f" looks more like a factor, than the function name. The way how the function is displayed in the blue circle is fine. I used the same code, basically, for both function names (simplified code):
\( f\left(\xi_{n,j}\right) \)
and
\( f(x) \)
Now, is there a way how to make the function identifier more "prominent", so it does not look like a factor to a parenthesis, but something more important I guess?
f(\xi_{n,j})(x_{n,j}-x_{n,j-1})
is just what's needed.