What is a lemma?(adsbygoogle = window.adsbygoogle || []).push({});

I hear people talking about "lemmas" all the time. A great example is the math joke that ends with the devil saying "But I found this really interesting lemma..." This joke would likely be much funnier to me if I knew what a lemma (or Riemann's hypothesis, but if you want to, I don't necessarily need to know that) is.

All help appreciated.

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# What is a lemma?

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