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In mathematics, a **closed manifold** is a manifold without boundary that is compact.
In comparison, an **open manifold** is a manifold without boundary that has only *non-compact* components.

The only connected one-dimensional example is a circle.
The sphere, torus, and the Klein bottle are all closed two-dimensional manifolds. The real projective space **RP**^{n} is a closed n-dimensional manifold. The complex projective space **CP**^{n} is a closed 2n-dimensional manifold.^{[1]}
A line is not closed because it is not compact.
A closed disk is a compact two-dimensional manifold, but it is not closed because it has a boundary.

Every closed manifold is a Euclidean neighborhood retract and thus has finitely generated homology groups.^{[2]}

If is a closed connected n-manifold, the n-th homology group is or 0 depending on whether is orientable or not.^{[3]} Moreover, the torsion subgroup of the (n-1)-th homology group is 0 or depending on whether is orientable or not. This follows from an application of the universal coefficient theorem.^{[4]}

Let be a commutative ring. For -orientable with
fundamental class , the map defined by is an isomorphism for all k. This is the Poincaré duality.^{[5]} In particular, every closed manifold is -orientable. So there is always an isomorphism .

For a connected manifold, "open" is equivalent to "without boundary and non-compact", but for a disconnected manifold, open is stronger. For instance, the disjoint union of a circle and a line is non-compact since a line is non-compact, but this is not an open manifold since the circle (one of its components) is compact.

Most books generally define a manifold as a space that is, locally, homeomorphic to Euclidean space (along with some other technical conditions), thus by this definition a manifold does not include its boundary when it is embedded in a larger space. However, this definition doesn’t cover some basic objects such as a closed disk, so authors sometimes define a manifold with boundary and abusively say *manifold* without reference to the boundary. But normally, a **compact manifold** (compact with respect to its underlying topology) can synonymously be used for **closed manifold** if the usual definition for manifold is used.

The notion of a closed manifold is unrelated to that of a closed set. A line is a closed subset of the plane, and a manifold, but not a closed manifold.

The notion of a "closed universe" can refer to the universe being a closed manifold but more likely refers to the universe being a manifold of constant positive Ricci curvature.