# Interpretation of f'(x)/f(x)

1. Jan 24, 2016

### Bruno Tolentino

Hi! I'd like to know of f'(x)/f(x) has some special interpretation, some physic or math concept related.

This ratio appears many times in control theory...

2. Jan 24, 2016

### BvU

Hehe, no idea. However, the inverse is where a linear approximation to f crosses the y-axis.

3. Jan 24, 2016

### Staff: Mentor

Since its anti-derivative is $ln|f(x)|$ it's natural to often occur everywhere. However, that doesn't justify a special name. E.g. $e^{- \frac{1}{2} x^2}$ hasn't either.

4. Jan 25, 2016

### Ssnow

It is the derivative of $\log{(f)}$, $\log$-transformations are used when there are quantities with exponential growth as in biology, control theory, in information theory as example $\log{f}$ is connected to the concept of entropy, ...

5. Jan 25, 2016

### Svein

In complex analysis, if f(z) is a meromorphic function inside and on some closed contour C, and f has no zeros or poles on C, then

where N and P denote respectively the number of zeros and poles of f(z) inside the contour C, with each zero and pole counted as many times as its multiplicity and order, respectively, indicate. This statement of the theorem assumes that the contour C is simple, that is, without self-intersections, and that it is oriented counter-clockwise (see https://en.wikipedia.org/wiki/Argument_principle).