I'm writing Android app.
The problem is that it should execute some calculations and library for this is written in Python.
What is the best way to invoke Python from Android/Java?
I heard about Kivy and even managed to run application, but python code returns latex formulae, that can't be rendered within Kivy app.
You can probably render LaTeX in Kivy fairly easily using a png exporter (such as presumably exists for web export tools, and modes like emacs' preview mode).
If you need to run python as part of a java app, probably a practical way to do it is to use kivy's python-for-android tools with your own java frontend, invoking the python interpreter via JNI. This would require some thinking and experimentation but should be possible. There are also other projects for building python for android, which might be able to do the same things.
Run Python Code from Android