Answer to Question #148126 in Discrete Mathematics for Promise Omiponle

Question #148126
(a) Show that a subset of a countable set is countable.
1
Expert's answer
2020-12-17T08:26:28-0500

Let BB be a subset of a countable set AA. By defenition of a countable subset, there is an injection f:ANf:A\to \mathbb N. Since the restriction fB:BNf|_B:B\to\mathbb N of an injection ff to the subset BB is an injection as well, we conclude that BB is a countable set.



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