*For hyper-connectivity in node-link graphs, see Connectivity_(graph_theory)#Super-_and_hyper-connectivity.*

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In the mathematical field of topology, a **hyperconnected space**^{[1]} or **irreducible space**^{[2]} is a topological space *X* that cannot be written as the union of two proper closed sets (whether disjoint or non-disjoint). The name *irreducible space* is preferred in algebraic geometry.

For a topological space *X* the following conditions are equivalent:

- No two nonempty open sets are disjoint.
*X*cannot be written as the union of two proper closed sets.- Every nonempty open set is dense in
*X*. - The interior of every proper closed set is empty.
- Every subset is dense or nowhere dense in
*X*.

A space which satisfies any one of these conditions is called *hyperconnected* or *irreducible*.

An **irreducible set** is a subset of a topological space for which the subspace topology is irreducible. Some authors do not consider the empty set to be irreducible (even though it vacuously satisfies the above conditions).

## Examples

Two examples of hyperconnected spaces from point set topology are the cofinite topology on any infinite set and the right order topology on .

In algebraic geometry, taking the spectrum of a ring whose reduced ring is an integral domain is an irreducible topological space—applying the lattice theorem to the nilradical, which is within every prime, to show the spectrum of the quotient map is a homeomorphism, this reduces to the irreducibility of the spectrum of an integral domain. For example, the schemes

,

are irreducible since in both cases the polynomials defining the ideal are irreducible polynomials (meaning they have no non-trivial factorization). A non-example is given by the normal crossing divisor

since the underlying space is the union of the affine planes , , and . Another non-example is given by the scheme

where is an irreducible degree 4 homogeneous polynomial. This is the union of the two genus 3 curves (by the genus–degree formula)

## Hyperconnectedness vs. connectedness

Every hyperconnected space is both connected and locally connected (though not necessarily path-connected or locally path-connected).

Note that in the definition of hyper-connectedness, the closed sets don't have to be disjoint. This is in contrast to the definition of connectedness, in which the open sets are disjoint.

For example, the space of real numbers with the standard topology is connected but *not* hyperconnected. This is because it cannot be written as a union of two disjoint open sets, but it *can* be written as a union of two (non-disjoint) closed sets.

## Properties

- The (non-empty) open subsets of a hyperconnected space are "large" in the sense that each one is dense in
*X*and any pair of them intersects. Thus, a hyperconnected space cannot be Hausdorff unless it contains only a single point.

- Every hyperconnected space is both connected and locally connected (though not necessarily path-connected or locally path-connected).

- Since the closure of every non-empty open set in a hyperconnected space is the whole space, which is an open set, every hyperconnected space is extremally disconnected.

- The continuous image of a hyperconnected space is hyperconnected.
^{[3]}In particular, any continuous function from a hyperconnected space to a Hausdorff space must be constant. It follows that every hyperconnected space is pseudocompact.

- Every open subspace of a hyperconnected space is hyperconnected.
^{[4]}

- Proof:
*Let be an open subset. Any two disjoint open subsets of would themselves be disjoint open subsets of . So at least one of them must be empty.*

- More generally, every dense subset of a hyperconnected space is hyperconnected.

- Proof:
*Suppose is a dense subset of and with , closed in . Then . Since is hyperconnected, one of the two closures is the whole space , say . This implies that is dense in , and since it is closed in , it must be equal to .*

- A closed subspace of a hyperconnected space need not be hyperconnected.

- Counterexample:
*with an algebraically closed field (thus infinite) is hyperconnected*^{[5]}in the Zariski topology, while is closed and not hyperconnected.

- The closure of any irreducible set is irreducible.
^{[6]}

- Proof:
*Suppose where is irreducible and write for two closed subsets (and thus in ). are closed in and which implies or , but then or by definition of closure.*

- A space which can be written as with open and irreducible such that is irreducible.
^{[7]}

- Proof:
*Firstly, we notice that if is a non-empty open set in then it intersects both and ; indeed, suppose , then is dense in , thus and is a point of closure of which implies and a fortiori . Now and taking the closure therefore is a non-empty open and dense subset of . Since this is true for every non-empty open subset, is irreducible.*

## Irreducible components

An **irreducible component**^{[8]} in a topological space is a maximal irreducible subset (i.e. an irreducible set that is not contained in any larger irreducible set). The irreducible components are always closed.

Every irreducible subset of a space *X* is contained in a (not necessarily unique) irreducible component of *X*.^{[9]} In particular, every point of *X* is contained in some irreducible component of *X*. Unlike the connected components of a space, the irreducible components need not be disjoint (i.e. they need not form a partition). In general, the irreducible components will overlap.

The irreducible components of a Hausdorff space are just the singleton sets.

Since every irreducible space is connected, the irreducible components will always lie in the connected components.

Every Noetherian topological space has finitely many irreducible components.^{[10]}

## See also

## Notes

**^**Steen & Seebach, p. 29**^**https://stacks.math.columbia.edu/tag/004U**^**Bourbaki, Nicolas (1989).*Commutative Algebra: Chapters 1-7*. Springer. p. 95. ISBN 978-3-540-64239-8.**^**Bourbaki, Nicolas (1989).*Commutative Algebra: Chapters 1-7*. Springer. p. 95. ISBN 978-3-540-64239-8.**^**Perrin, Daniel (2008).*Algebraic Geometry. An introduction*. Springer. p. 14. ISBN 978-1-84800-055-1.**^**https://stacks.math.columbia.edu/tag/004W**^**Bourbaki, Nicolas (1989).*Commutative Algebra: Chapters 1-7*. Springer. p. 95. ISBN 978-3-540-64239-8.**^**https://stacks.math.columbia.edu/tag/004V**^**https://stacks.math.columbia.edu/tag/004W**^**https://stacks.math.columbia.edu/tag/0050

## References

- Steen, Lynn Arthur; Seebach, J. Arthur Jr. (1995) [1978],
*Counterexamples in Topology*(Dover reprint of 1978 ed.), Berlin, New York: Springer-Verlag, ISBN 978-0-486-68735-3, MR 0507446 - "Hyperconnected space".
*PlanetMath*.