suppose $X$ is a set and $e$, $f$ are two metric on the set $X$. Then I knew that $$(x,y)\rightarrow \min\{e(x,y),f(x,y)\}$$ is not a metric on $X$. There are counterexamples in which triangle inequality fails. But I want to know under what condition this will be a metric on $X$.
Thanks for help in advance.