- It is well known that between a prime $p$ and $p^2$ always exist a prime, but what is the shortest proof of that (by elementary methods or not)?
(One can say that we can have it as a collorary of Bertrand's postulate, but it is a stronger result.)
I ask it as an example, are there any results that their first proof was really large comparing to some proof that someone found later?
Or of results that their only known proof/proofs is/are a collorary of the proof of something stronger? (Maybe the one that I give is not an example for this.)
NOTE:I asked to change the title to a more precise