Sometimes people declare redundant axioms. Positivity is one of them.
Chinese is a fascinating language. What about Chimese, though?
I've never seen a proof this beautiful for the countability of Q.
About my discussion with a professor in the final round of the Bundeswettbewerb Mathematik.
Do you want to understand math? Then draw a picture.
On redundant axioms such as commutativity.
Recursive thinking is inductive thinking. I've always struggled with recursion. Until I've understood how to really think about it.