Where did Lagrange prove that if $p\equiv 3\bmod{4}$ and $q=2p+1$ are primes, then $q$ divides $2^p-1$?

113 Views Asked by Bumbble Comm At 13 Apr 2026 - 4:19

It is said that Lagrange proved in 1775 that

if $p\equiv 3\bmod{4}$ and $q=2p+1$ are primes, then $q$ divides $2^p-1$

but I have not been able to find the source, where he did this. Can you help me?

Edit: This is said in many places, for example, at