Cut point

In topology, a cut-point is a point of a connected space such that its removal causes the resulting space to be disconnected. If removal of a point doesn't result in disconnected spaces, this point is called a non-cut point.

The "neck" of this eight-like figure is a cut-point.

For example, every point of a line is a cut-point, while no point of a circle is a cut-point.

Cut-points are useful to determine whether two connected spaces are homeomorphic by counting the number of cut-points in each space. If two spaces have different number of cut-points, they are not homeomorphic. A classic example is using cut-points to show that lines and circles are not homeomorphic.

Cut-points are also useful in the characterization of topological continua, a class of spaces which combine the properties of compactness and connectedness and include many familiar spaces such as the unit interval, the circle, and the torus.

Definition

Formal definitions

A line (closed interval) has infinitely many cut points between the two endpoints. A circle has no cut point. Since they have different numbers of cut points, lines are not homeomorphic to circles.

A point ${\displaystyle p}$ of a connected topological space ${\displaystyle X}$ is called a cut point^[1][2] of ${\displaystyle X}$ if ${\displaystyle X\setminus \{p\}}$ is not connected. A point ${\displaystyle p}$ of a connected space ${\displaystyle X}$ is called a non-cut point^[1] of ${\displaystyle X}$ if ${\displaystyle X\setminus \{p\}}$ is connected.

Note that these two notions only make sense if the space ${\displaystyle X}$ is connected to start with. Also, for a space to have a cut point, the space must have at least three points, because removing a point from a space with one or two elements always leaves a connected space.

A non-empty connected topological space X is called a cut-point space^[2] if every point in X is a cut point of X.

Basic examples

• A closed interval [a,b] has infinitely many cut points. All points except for its endpoints are cut points and the endpoints {a,b} are non-cut points.
• An open interval (a,b) has infinitely many cut points, like closed intervals. Since open intervals don't have endpoints, it has no non-cut point.
• A circle has no cut point. Every point of a circle is a non-cut point.

Notations

• A cutting of X is a set {p,U,V} where p is a cut-point of X, U and V form a separation of X-{p}.
• Also can be written as X\{p}=U|V.

Theorems

Cut-points and homeomorphisms

• Cut-points are not necessarily preserved under continuous functions. For example: f: [0, 2π] → R², given by f(x) = (cos x, sin x). Every point of the interval (except the two endpoints) is a cut-point, but f(x) forms a circle which has no cut-points.
• Cut-points are preserved under homeomorphisms. Therefore, cut-point is a topological invariant.

Cut-points and continua

• Every continuum (compact connected Hausdorff space) with more than one point, has at least two non-cut points. Specifically, each open set which forms a separation of resulting space contains at least one non-cut point.
• Every continuum with exactly two noncut-points is homeomorphic to the unit interval.
• If K is a continuum with points a,b and K-{a,b} isn't connected, K is homeomorphic to the unit circle.

Topological properties of cut-point spaces

• Let X be a connected space and x be a cut point in X such that X\{x}=A|B. Then {x} is either open or closed. if {x} is open, A and B are closed. If {x} is closed, A and B are open.
• Let X be a cut-point space. The set of closed points of X is infinite.

Irreducible cut-point spaces

Definitions

A cut-point space is irreducible if no proper subset of it is a cut-point space.

The Khalimsky line: Let ${\displaystyle \mathbb {Z} }$ be the set of the integers and ${\displaystyle B=\{\{2i-1,2i,2i+1\}:i\in \mathbb {Z} \}\cup \{\{2i+1\}:i\in \mathbb {Z} \}}$ where ${\displaystyle B}$ is a basis for a topology on ${\displaystyle \mathbb {Z} }$. The Khalimsky line is the set ${\displaystyle \mathbb {Z} }$ endowed with this topology. It's a cut-point space. Moreover, it's irreducible.

Theorem

• A topological space ${\displaystyle X}$ is an irreducible cut-point space if and only if X is homeomorphic to the Khalimsky line.

See also

Cut point (graph theory)