1. The problem statement, all variables and given/known data Prove that the open interval (1,3) and the closed interval [1,4] have the same cardinality. 2. Relevant equations 3. The attempt at a solution I have to prove bijection. The injective part is obvious. Say, A =(1,3) and B =[1,4] f: A → B f(x) = x It's injective. I have problems with g: B → A g(x) = 1+ ... ? Help. I plan to use Cantor, Schroder, and Bernstein's theorem.