I once read that some mathematicians provided a very length proof of 1+1=2

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Can you think of some way to extend mathematical rigor to present a long proof of that equation? I’m not asking for a proof, but rather for some outline what one would consider to make the derivation as long as possible.

EDIT: It seems the proof I heard about is a standard reference given here multiple times ðŸ™‚ I stated that the proof itself is less useful than an outline for me as I know only “physics level maths”. Can someone provide a short outline what’s going on in the proof? Some outline I can look up section by section in Wikipedia to at least get a feel of what could be needed to make such a proof?

I suspect you are referring to the Principia Mathematica. I direct you to a quotation from Wikipedia about how the proof doesn’t appear until page 379.