Hello! This is more of a set theory question I guess, but I have that the definition of the boundary of a subset A of a topological space X is ##\partial A = \bar A \cap \bar B##, with ##B = X - A## (I didn't manage to put the bar over X-A, this is why I used B). I think I have a wrong understanding of the complement of a set because if I take (a,b) on the real axis, the boundary should be {a,b}, but ##\bar A = (- \infty, a] \cup [b, \infty)## while ##B=R-A = (- \infty, a] \cup [b, \infty)## so ##\bar B = (a,b)## and ##\bar A \cap \bar B = \emptyset##. So where exactly I got it wrong? Thank you(adsbygoogle = window.adsbygoogle || []).push({});

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# I Definition of boundary

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