3.I.4A
Show that a topology is determined on the real line by specifying that a nonempty subset is open if and only if it is a union of half-open intervals , where are real numbers. Determine whether is Hausdorff.
Let denote the cofinite topology on (that is, a non-empty subset is open if and only if its complement is finite). Prove that the identity map induces a continuous .
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