Does someone know what's meant by a binary open cover of a topological space $X$? I can't find this definition of binary open cover. Could someone who knows it tell me? Thanks ahead for any help:)
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A binary open cover of a space $X$ is simply an open cover with exactly two members. For example, a $T_1$ space is normal iff every binary (= two-element) open cover of $X$ has a binary closed refinement. (This is an easy exercise.)