Elementary theory of prime numbers

Chebychev's Theorem states that the order of magnitude of the prime counting function $\pi(x)$ is $x/\log x$. This result was "the state of the art" until the proof of the Prime Number Theorem by Hadamard and de la Vallée-Poussin in 1895. We shall outline a proof of Chebychev's estimates and deduce from his estimate Mertens' Theorem which provides an asymptotic formula for $\sum_{p\le x}p^{-1}$ as $x\rightarrow\infty$.