Def 13.1 (𝑋,𝑑) is called metric space 0) 𝑋 is set 𝑑:𝑋×𝑋⟶[0,𝛼) 𝑑(𝑥,𝑦)=𝑑(𝑦,𝑥) 𝑑(𝑥,𝑦)=0⟺𝑥=𝑦 𝑑(𝑥,𝑦)≤𝑑(𝑥,𝑧)+𝑑(𝑧,𝑦) 3 standard choices of distance function 17.4 𝑑∞(𝑥,𝑦)=max𝑑|𝑥(𝑑)−𝑦(𝑑)| 𝑑2(𝑥,𝑦)=√∑𝑑|𝑥(𝑑)−𝑦(𝑑)|2 𝑑1(𝑥,𝑦)=∑𝑑𝑑=1|𝑥(𝑑)−𝑦(𝑑)|