What is the name of this type of "average"?

Question

Given two variables $x$ and $y$, $$\frac{2}{1/x + 1/y} $$ looks like some kind of average of $x$ and $y$. Is there a name for it?

Where can i look up more information (such as properties) of this type of average?

Answer 1

This is the harmonic mean:

$$\frac{n}{\sum_{i=1}^n \frac{1}{a_i}} $$

See also: https://en.wikipedia.org/wiki/Harmonic_mean