I know this is a long short but I was wondering whether it is possible to substitute a combination of variables by something else. Let me explain it with an example.
Let f(x,y,z) = f(x) \sqrt(x^2+y^2+z^2) + f(y) \sqrt(x^2+y^2+z^2) + f(z) \sqrt(x^2+y^2+z^2)
As we can see the combination \sqrt(x^2+y^2+z^2) occurs very often. Is there a way I call this combination some other variable, say, w. I cannot use substitute in this case as I want to leave x,y and z alone if they dont come in this specific combination \sqrt(x^2+y^2+z^2). If it were a string I could have used find and replace. But I want to use the expression as a input later on in the notebook.
Thanks in advance.