I've seen this assertion in a few comments around the site, and I found the answer to the $\rightarrow$ implication here.
Does anyone know a (hopefully simple) proof of the $\leftarrow$ implication?
I've seen this assertion in a few comments around the site, and I found the answer to the $\rightarrow$ implication here.
Does anyone know a (hopefully simple) proof of the $\leftarrow$ implication?