Since people are randomly listing various combinations of `\\` with other characters, let me for the record repeat here what I wrote in the comments to https://meta.mathoverflow.net/q/238:

* *Every* `\\` needs to be simplified to `\` unless it appears in a `\substack` or inside a matrix or array-like LaTeX environment (e.g., matrix, align, gather, cases), where it could be legitimately used as a line break. What follows the `\\` is otherwise irrelevant, though as a particular case, `\\\` or `\\\\` should yield `\\`.