A noncompact manifold without boundary.
Open Manifold
See also
Closed Manifold, Compact Manifold, ManifoldThis entry contributed by Todd Rowland
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Rowland, Todd. "Open Manifold." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/OpenManifold.html