consider (ℝ,τ) , where τ={∅}∪{u⊆ℝ:{-π,π}⊆u}.(adsbygoogle = window.adsbygoogle || []).push({});

Is [-4.4] open in (ℝ,τ) ? Justify your answer

Is (-3,3) closed in (ℝ,τ) ? Justify your answer

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# Open and closed

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