Bertrand's Postulate - For ∀n∈{1,2,3,...}, ∃prime p∈{n+1, n+2, ..., 2n}

Bertrand's postulate, also called the Bertrand-Chebyshev theorem or Chebyshev's theorem.
The video follows the proof in Proofs from THE BOOK (Aigner and Ziegler):