Bertrand-Chebyshev theorem

Etymology
From the surname of Joseph Louis François Bertrand, who conjectured the theorem, and the surname of Pafnuty Chebyshev who proved it.

Proper noun

 * 1)  the theorem that there is at least one prime number between n and 2n for every n>1, i.e.:
 * $$\forall n\in\mathbb{N}:n>1\Rightarrow\exists p\in\mathbb{P}:n<p<2n$$