I can see that the post here display nicely typeset mathematical formulas. How can I get such formulas in my posts? What syntax is used to typeset them?

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Note: This is an attempt to make at least brief overview of MathJax syntax as suggested some time ago in this answer: [FAQ on typesetting of formulae hard to find](https://meta.mathoverflow.net/q/2957#2958). The question and answers are [community wiki](https://meta.mathoverflow.net/tags/community-wiki/info) in order to encourage collaborative editing. We should probably try to keep the tutorial simple and brief - with links to other resources, in case somebody needs more details.