This may seem like an overly trivial question, but I've just recently become confused about Langrange's 'prime' notation for derivatives (for example $f'(x)$).
I know for sure that f'(x) = \frac{\delta f(x)}{\delta x}.
But suppose we replace x with an expression, like 2x+1. Do we write f'(x^2+1) = \frac{\delta f(x^2+1)}{\delta x} or f'(x^2+1) = \frac{\delta f(x^2+1)}{\delta (x^2+1)}?
Does putting the prime around the function instead of between its letter and parentheses make a difference? For example what does (f(x^2+1))' mean?