Texer Aops File

It looks like you're asking about on the Art of Problem Solving (AoPS) website.

Here’s a concise guide covering what it is, how to access it, and basic syntax. TeXer is AoPS's built-in LaTeX editor and previewer. It lets you write mathematical expressions and see exactly how they will render in forum posts, messages, or contest solutions—without posting first. texer aops