效绵鼎 Countable and Uncountable Sets the natural numbers N={1,2,3,...}are countable Definition:a set S is countable if it is finite,or it is infinite and there is a bijection f:N→SCountable and Uncountable Sets ◼ the natural numbers N = {1,2,3,…} are countable ◼ Definition: a set S is countable if it is finite, or it is infinite and there is a bijection f: N → S