Why is Bertrand's Postulate Called a "Postulate?"

It is a theorem since a long time. Bertrand conjectured it (in 1845) but did not prove it.

Postulate means something that is assumed to be true. As an axiom or tentatively as a conjecture. As detailed in a comment by Mauro Allegranza this matches exactly what happened. That is, Betrand assumed this to be true (tentatively as a conjecture) in order to be able to progress with an argument.

It seems he did not use the word himself though. Yet, Chebyshev already used this term few years later (1852) writing in the paper where he proved it "ce qui est le postulatum connu de M. Bertrand" meaning "which is the known postulatum of Mr. Betrand" (my translation).


"Postulate" is used here as a synonym of "conjecture". Bertrand conjectured this in 1845, Chebyshev proved it in 1852.

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