Description: If a topology is second-countable, every base has a countable subset which is a base. Exercise 16B2 in Willard. (Contributed by Jeff Hankins, 28-Jan-2010) (Proof shortened by Mario Carneiro, 21-Mar-2015)
Ref | Expression | ||
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Hypotheses | 2ndcctbss.1 | |
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2ndcctbss.2 | |
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Assertion | 2ndcctbss | |