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. It's not as formal as it could be, but I'm not a pure mathematician by a long way.
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Don't be so humble, I think the proof is really good.
Now, about my attempt for 1. Consider the square to be ranging from -2 to 2 on every axis. The sequence of centers then satifies

, so it is a Cauchy sequence.
For 3, induction gives an answer, but I don't like that kind of proof

Let me look if I can find something better.
For 2, I am at a loss

Hacker's love for algebra has ruined him!!