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• 159 Why don't we define "imaginary" numbers for every "impossibility"?
• 54 Fun but serious mathematics books to gift advanced undergraduates.
• 52 If $f(x)\leq f(f(x))$ for all $x$, is $x\leq f(x)$?
• 49 We all use mathematical induction to prove results, but is there a proof of mathematical induction itself?
• 45 Prove that the union of countably many countable sets is countable.
• 41 In proofs, are "for each" and "for any" synonyms?
• 37 Implies ($\Rightarrow$) vs. Entails ($\models$) vs. Provable ($\vdash$)
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