I'm working on a project involving synthesizing audio, which led me to a promising Stanford CCRMA paper on band-limited synthesis. However, this double-arrow notation keeps popping up throughout some pretty critical portions of the document, and I've been unable to find any mention of it in the paper itself (you can imagine running a Google search for this might be tricky). It seems as though it has something to do with sampling/resampling (as in sampling the continuous variable $t$ at a rate of $nT_s$. But this doesn't seem to explain it's use in $(\leftrightarrow 1)^n$ (for lack of a better symbol). I'm hoping someone here might be able to shed some insight.
Here's a link to the paper for further examples: https://ccrma.stanford.edu/~stilti/papers/blit.pdf