Talk:Square root of 2

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There are only 115 propositions in book X of Elements

The article cites the proof of irrationality to Elements book X proposition 117, which doesn't exist.  Preceding unsigned comment added by 2601:647:C901:20C0:28AA:3E0A:5D6A:B040 (talk) 22:16, 21 August 2024 (UTC)

It exists, it is merely often numbered differently, because the consensus of scholars is that it is a later addition to Euclid (by other ancient Greek mathematicians): see . —David Eppstein (talk) 23:02, 21 August 2024 (UTC)
Our discussion about the general topic of the Elements Book X, incommensurability, the Greek concept(s) of ratio and proportion, etc., could be much more complete. There are a couple of books by Knorr (1975) and Fowler (1987) as well as various papers by these authors and others, discussing the pre-Euclidean history, and there is also a long post-Euclidean history, none of which we do a very good job describing anywhere in Wikipedia. –jacobolus (t) 00:47, 22 August 2024 (UTC)

Constructive validity of the usual proof that sqrt(2) is irrational

So the proof is essentially "to prove ¬p, assume p and reach some contradiction" with p being " is rational". This is of course constructive valid because p ⇒ false is exactly the definition of ¬p.

On the other hand, the proof by contradiction is "to prove p, assume ¬p and reach some contradiction". This is not constructive valid because what we proved is, by definition, ¬¬p.

In practice we often call both "assume p to prove ¬p" and "assume ¬p to prove p" proof by contradiction, but if we want to be rigorous only the second one is the "real" proof by contradiction as the first one is constructive and logically uncontroversial. 129.104.241.224 (talk) 18:47, 2 December 2024 (UTC)

Proposed: two additional (√2) series representations

Constructive proof steps

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