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Homework Help: Proving a metric is continuous

  1. Sep 26, 2010 #1


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    Homework Helper

    1. The problem statement, all variables and given/known data

    So, given a metric d : X x X --> R, prove that d is continuous.

    3. The attempt at a solution

    Let (x, y) be a point in X x X, V = <a, b> a neighborhood of d(x, y). One needs to find a neighborhood of U of (x, y) such that d(U) is contained in V. U is of the form U1 x U2, where U1 is a neighborhood of x, and U2 a neighborhood of y. I claim that every union U of two open balls B1(x, r1) and B2(y, r2), where 2(r1 + r2) = b - a, must satisfy d(U) [tex]\subseteq[/tex] <a, b>.

    The diameter of B1 is 2r1, and the diameter of B2 is 2r2. The diameter of their union is b = a + 2r1 + 2r2, where a is the distance between B1 and B2.

    Does this work?
  2. jcsd
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