The standard command for not equal is \neq, which produces ≠. Inequality commands include \le (≤) and \ge (≥), and these symbols are typically found in math mode only. In the categories listed by Overleaf, the inequality symbols fall under Binary Operation/Relation Symbols.
However, there are situations where you may prefer to use the \not prefix rather than simply applying it directly to an operator. This is primarily due to a difference in stroke angle between how some operators appear. If you're not convinced, refer to this discussion on TeX Stack Exchange where one user discusses a situation that arises when using the symbol \varnothing, a set containing no elements, represented as ∅. Here, you might want to match the slope of slanted vertical lines used for other symbols like the less-than or greater-than signs, both of which have a negative slope. The solution would be to construct your own variant, such as $$\not= $$. However, if you don’t care about aesthetics, you can apply \not to the default operator.
The rendering bug
A few weeks ago, we reported that Notesnook had a problem rendering certain mathematical expressions correctly. Since then, we’ve received similar reports about Notesnook from readers who said that they saw the same issue with the “$$” environment, but didn’t report that Notesnook was failing to render them correctly.
We did find something interesting, though: a bug appears when using the equation editor provided by a desktop app. It doesn’t affect code blocks or plain text. Instead, when we rendered the following in an equation:
$$\neq$$
Notesnook displayed the symbol as just an ordinary equal sign. However, this does appear to be a true rendering bug, not simply a problem with the way the notes are formatted.
The zoom fix
Overleaf, which hosts many LaTeX documents, can also suffer from issues caused by browser zoom settings. For example, here’s what appears in our document using the equation editor on Overleaf. It shows up fine without any issues:
Here’s another example from the Overleaf documentation site. But the issue appears to be caused by browser zoom. If you reduce your browser zoom setting to 75%, the equation appears to be correct. If you increase it to 100%, however, the two lines of the equal sign seem to appear at different levels, as shown below. However, if you increase the zoom to 200%, the issue goes away.
But why does this occur? According to this article from Overleaf, the reason is simple: if you’re using the Overleaf service, it runs within a browser. If your browser is zoomed in, you’ll see the effect.