Try doing some basic maths questions in the Lean Theorem Prover. Functions, real numbers, equivalence relations and groups. Click on README.md and then on "Open in CoCalc with one click".

Hints for challenge 2.

You can always unfold something to see its definition. For example you can do unfold is_lub at ha and then you can unfold the new things which appear.

The theorem that if a ≤ b and b ≤ a then a = b is called le_antisymm.