Description: The basis for the product topology can also be written as the set of finite intersections of "cylinder sets", the preimages of projections into one factor from open sets in the factor. (We have to add X itself to the list because if A is empty we get ( fi(/) ) = (/) while B = { (/) } .) (Contributed by Mario Carneiro, 3-Feb-2015)
Ref | Expression | ||
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Hypotheses | ptbas.1 | |
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ptbasfi.2 | |
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Assertion | ptbasfi | |