AxiomNote (Axiom + Note) is a free and open source project that aims to provide an complete but easy to use note-taking app for all your mathematical needs!
With AxiomNote, you can write and read Markdown files, with the added bonus of out-of-the-box KaTeX integration! With this, you can easily write mathematical expressions.
Of course you have your standard suite of features a note-taking app has, such as file searching (not finished), word finder (not finished), etc...
When finding a bug, just create an issue explaining how to reproduce it and what you're tring to achieve. If you feel like you can fix it yourself, you can always make a PR and help me out!
You can contribute in a few ways. The most direct way of helping me is fixing issues other users have, either by answering them or directly sending a PR with a fix. However, you can also help by translating the app to your native language, translating the docs, fixing typos, etc...
Another way to directly support me is by donating whatever you feel like by clicking the little sponsor button! This is not in any way mandatory, but it would help out a ton! I'm just a highschool student, after all.
Whatever you want. I'm not picky about what you do with the source code. Make your own app with it. Pick it apart, study it, repackage it and sell it. It's yours to do whatever you feel like.