Hausdorff reflections and bifurcate curves

Abstract

A manifold is a space that locally looks like the smooth space ${\mathbf{R}}^n$. It is usually also assumed that the underlying topological space of a manifold is hausdorff. However, there are natural examples of manifolds for which the hausdorff conditions fails. Some but not all of these examples contain bifurcate pairs of curves: pairs of curves that agree on some initial interval but disagree on a later interval. The first part of this note proves that a manifold $M$ is hausdorff if and only if (i) it contains no bifurcate curves and (ii) there is a hausdorff manifold $N$ with the same algebra of smooth real-valued functions as $M$; this confirms a conjecture of Wu and Weatherall. The second part of this note shows that a hausdorff manifold $N$ satisfying (ii) is a certain quotient of $M$.

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