Perhaps annoying clarification. The word closure has two important meanings.
In a topological space a set is closed if it is the complement of an open set. In the more concrete case of Rn, which we're talking about, a set is closed if it contains it's limit points (boundary). The other use of closure is the one Glen(?) mentioned. The sum of two elements in the set is in the set and the same for other operations. Barry, help! Frank ----------------------------------- Frank Wimberly My memoir: https://www.amazon.com/author/frankwimberly My scientific publications: https://www.researchgate.net/profile/Frank_Wimberly2 Phone (505) 670-9918
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