What does this double sided arrow $\longleftrightarrow$ mean? What is $\longleftrightarrow$ used for in mathematics? I know about $\iff$ being used for "If and only if". Are they the same thing? I was watching a YouTube video that said:
$$\sum^{\infty}_{n=1} {1\over n^x} \longleftrightarrow \int^{\infty}_{1} {1\over t^x} dt$$
The teacher mentions convergence/divergence, but I was confused when the notation came up.

As has been mentioned in the comments, this is almost certainly an idiosyncratic use, and the author (is that the right word for somebody who makes a YouTube video? Probably not) ought to have explained what he or she intended the symbol to mean.  Without any additional context, it's hard to know for sure, but I'm going to hazard a guess that the symbol is intended to denote "are equivalent" in some (perhaps ill-defined) sense.  In what sense?  Probably in the sense of "equiconvergence" -- i.e., their convergence behavior is equivalent (one of them converges if and only if the other one does).
