"\\text { Find the interior Int }\\left\\{\\frac{1}{n} \\mid n \\in \\mathbb{N}\\right\\} \\text { of a subset of } \\mathbb{R} \\text {. }" (DG)
interior of a subset S of a topological space X is the union of all subsets of S that are open in X.
let "S=\\{\\frac{1}{n}|n\\isin N\\}"
for every "x=1\/n\\isin S" , every neighborhood "N(1\/n,\\varepsilon)" contains irrational numbers (i.e. numbers not in S) so x is not an interior point. Thus, "Int\\ S=\\empty"
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