Brouwer’s fixedpoint theorem is a fixedpoint theorem in topology, named after Luitzen Brouwer. It states that for any continuous function
$$f
{\displaystyle f}
mapping a compact convex set into itself there is a point
$$x
0
{\displaystyle x_{0}}
such that
$$f
(
x
0
)
=
x
0
{\displaystyle f(x_{0})=x_{0}}
. The simplest forms of Brouwer’s theorem are for continuous functions
$$f
{\displaystyle f}
from a closed interval
$$I
{\displaystyle I}
in the real numbers to itself or from a closed disk
$$D
{\displaystyle D}
to itself. A more general form than the latter is for continuous functions from a convex compact subset
$$K
{\displaystyle K}
of Euclidean space to itself.
Among hundreds of fixedpoint theorems, Brouwer’s is particularly well known, due in part to its use across numerous fields of mathematics. In its original field, this result is one of the key theorems characterizing the topology of Euclidean spaces, along with the Jordan curve theorem, the hairy ball theorem and the Borsuk–Ulam theorem. This gives it a place among the fundamental theorems of topology. The theorem is also used for proving deep results about differential equations and is covered in most introductory courses on differential geometry. It appears in unlikely fields such as game theory. In economics, Brouwer’s fixedpoint theorem and its extension, the Kakutani fixedpoint theorem, play a central role in the proof of existence of general equilibrium in market economies as developed in the 1950s by economics Nobel prize winners Kenneth Arrow and Gérard Debreu.
The theorem was first studied in view of work on differential equations by the French mathematicians around Poincaré and Picard. Proving results such as the Poincaré–Bendixson theorem requires the use of topological methods. This work at the end of the 19th century opened into several successive versions of the theorem. The general case was first proved in 1910 by Jacques Hadamard and by Luitzen Egbertus Jan Brouwer.
The theorem has several formulations, depending on the context in which it is used and its degree of generalization. The simplest is sometimes given as follows:
This can be generalized to an arbitrary finite dimension:
A slightly more general version is as follows:
An even more general form is better known under a different name:
The theorem holds only for sets that are compact (thus, in particular, bounded and closed) and convex. The following examples show why these requirements are important.
Consider the function
which is a continuous function from
$$R
{\displaystyle \mathbb {R} }
to itself. As it shifts every point to the right, it cannot have a fixed point. Note that
$$R
{\displaystyle \mathbb {R} }
is convex and closed, but not bounded.
Consider the function
which is a continuous function from the open interval (−1,1) to itself. In this interval, it shifts every point to the right, so it cannot have a fixed point. Note that (−1,1) is convex and bounded, but not closed. The function f does have a fixed point for the closed interval [−1,1], namely f(x) = x = 1.
Note that convexity is not strictly necessary for BFPT. Because the properties involved (continuity, being a fixed point) are invariant under homeomorphisms, BFPT is equivalent to forms in which the domain is required to be a closed unit ball
$$D
n
{\displaystyle D^{n}}
. For the same reason it holds for every set that is homeomorphic to a closed ball (and therefore also closed, bounded, connected, without holes, etc.).
The following example shows that BFPT doesn’t work for domains with holes. Consider the following function, defined in polar coordinates:
which is a continuous function from the unit circle to itself. It rotates every point on the unit circle 45 degrees counterclockwise, so it cannot have a fixed point. Note that the unit circle is closed and bounded, but it has a hole (and so it is not convex). The function f does have a fixed point for the unit disc, since it takes the origin to itself.
A formal generalization of BFPT for „holefree“ domains can be derived from the Lefschetz fixedpoint theorem.
The continuous function in this theorem is not required to be bijective or even surjective.
The theorem has several „real world“ illustrations. Here are some examples.
1. Take two sheets of graph paper of equal size with coordinate systems on them, lay one flat on the table and crumple up (without ripping or tearing) the other one and place it, in any fashion, on top of the first so that the crumpled paper does not reach outside the flat one. There will then be at least one point of the crumpled sheet that lies directly above its corresponding point (i.e. the point with the same coordinates) of the flat sheet. This is a consequence of the n = 2 case of Brouwer’s theorem applied to the continuous map that assigns to the coordinates of every point of the crumpled sheet the coordinates of the point of the flat sheet immediately beneath it.
2. Take an ordinary map of a country, and suppose that that map is laid out on a table inside that country. There will always be a „You are Here“ point on the map which represents that same point in the country.
3. In three dimensions the consequence of the Brouwer fixedpoint theorem is that, no matter how much you stir a cocktail in a glass, when the liquid has come to rest, some point in the liquid will end up in exactly the same place in the glass as before you took any action, assuming that the final position of each point is a continuous function of its original position, that the liquid after stirring is contained within the space originally taken up by it, and that the glass (and stirred surface shape) maintain a convex volume. Ordering a cocktail shaken, not stirred defeats the convexity condition („shaking“ being defined as a dynamic series of nonconvex inertial containment states in the vacant headspace under a lid). In that case, the theorem would not apply, and thus all points of the liquid disposition are potentially displaced from the original state, mathematically proving the controversial quality of shaken martinis as potentially bettermixed.
The theorem is supposed to have originated from Brouwer’s observation of a cup of coffee. If one stirs to dissolve a lump of sugar, it appears there is always a point without motion. He drew the conclusion that at any moment, there is a point on the surface that is not moving. The fixed point is not necessarily the point that seems to be motionless, since the centre of the turbulence moves a little bit. The result is not intuitive, since the original fixed point may become mobile when another fixed point appears.
Brouwer is said to have added: „I can formulate this splendid result different, I take a horizontal sheet, and another identical one which I crumple, flatten and place on the other. Then a point of the crumpled sheet is in the same place as on the other sheet.“ Brouwer „flattens“ his sheet as with a flat iron, without removing the folds and wrinkles. This example is better than the coffee cup one as it shows that uniqueness of the fixed point may fail. This distinguishes Brouwer’s result from other fixedpoint theorems, such as Banach’s, that guarantee uniqueness.
In one dimension, the result is intuitive and easy to prove. The continuous function f is defined on a closed interval [a, b] and takes values in the same interval. Saying that this function has a fixed point amounts to saying that its graph (dark green in the figure on the right) intersects that of the function defined on the same interval [a, b] which maps x to x (light green).
Intuitively, any continuous line from the left edge of the square to the right edge must necessarily intersect the green diagonal. Proof: consider the function g which maps x to f(x) – x. It is ≥ 0 on a and ≤ 0 on b. By the intermediate value theorem, g has a zero in [a, b]; this zero is a fixed point.
Brouwer is said to have expressed this as follows: „Instead of examining a surface, we will prove the theorem about a piece of string. Let us begin with the string in an unfolded state, then refold it. Let us flatten the refolded string. Again a point of the string has not changed its position with respect to its original position on the unfolded string.“
The Brouwer fixed point theorem was one of the early achievements of algebraic topology, and is the basis of more general fixed point theorems which are important in functional analysis. The case n = 3 first was proved by Piers Bohl in 1904 (published in Journal für die reine und angewandte Mathematik). It was later proved by L. E. J. Brouwer in 1909. Jacques Hadamard proved the general case in 1910, and Brouwer found a different proof in the same year. Since these early proofs were all nonconstructive indirect proofs, they ran contrary to Brouwer’s intuitionist ideals. Methods to construct (approximations to) fixed points guaranteed by Brouwer’s theorem are now known.
To understand the prehistory of Brouwer’s fixed point theorem one needs to pass through differential equations. At the end of the 19th century, the old problem of the stability of the solar system returned into the focus of the mathematical community. Its solution required new methods. As noted by Henri Poincaré, who worked on the threebody problem, there is no hope to find an exact solution: „Nothing is more proper to give us an idea of the hardness of the threebody problem, and generally of all problems of Dynamics where there is no uniform integral and the Bohlin series diverge.“ He also noted that the search for an approximate solution is no more efficient: „the more we seek to obtain precise approximations, the more the result will diverge towards an increasing imprecision“.
He studied a question analogous to that of the surface movement in a cup of coffee. What can we say, in general, about the trajectories on a surface animated by a constant flow? Poincaré discovered that the answer can be found in what we now call the topological properties in the area containing the trajectory. If this area is compact, i.e. both closed and bounded, then the trajectory either becomes stationary, or it approaches a limit cycle. Poincaré went further; if the area is of the same kind as a disk, as is the case for the cup of coffee, there must necessarily be a fixed point. This fixed point is invariant under all functions which associate to each point of the original surface its position after a short time interval t. If the area is a circular band, or if it is not closed, then this is not necessarily the case.
To understand differential equations better, a new branch of mathematics was born. Poincaré called it analysis situs. The French Encyclopædia Universalis defines it as the branch which „treats the properties of an object that are invariant if it is deformed in any continuous way, without tearing“. In 1886, Poincaré proved a result that is equivalent to Brouwer’s fixedpoint theorem, although the connection with the subject of this article was not yet apparent. A little later, he developed one of the fundamental tools for better understanding the analysis situs, now known as the fundamental group or sometimes the Poincaré group. This method can be used for a very compact proof of the theorem under discussion.
Poincaré’s method was analogous to that of Émile Picard, a contemporary mathematician who generalized the Cauchy–Lipschitz theorem. Picard’s approach is based on a result that would later be formalised by another fixedpoint theorem, named after Banach. Instead of the topological properties of the domain, this theorem uses the fact that the function in question is a contraction.
At the dawn of the 20th century, the interest in analysis situs did not stay unnoticed. However, the necessity of a theorem equivalent to the one discussed in this article was not yet evident. Piers Bohl, a Latvian mathematician, applied topological methods to the study of differential equations. In 1904 he proved the threedimensional case of our theorem, but his publication was not noticed.
It was Brouwer, finally, who gave the theorem its first patent of nobility. His goals were different from those of Poincaré. This mathematician was inspired by the foundations of mathematics, especially mathematical logic and topology. His initial interest lay in an attempt to solve Hilbert’s fifth problem. In 1909, during a voyage to Paris, he met Poincaré, Hadamard, and Borel. The ensuing discussions convinced Brouwer of the importance of a better understanding of Euclidean spaces, and were the origin of a fruitful exchange of letters with Hadamard. For the next four years, he concentrated on the proof of certain great theorems on this question. In 1912 he proved the hairy ball theorem for the twodimensional sphere, as well as the fact that every continuous map from the twodimensional ball to itself has a fixed point. These two results in themselves were not really new. As Hadamard observed, Poincaré had shown a theorem equivalent to the hairy ball theorem. The revolutionary aspect of Brouwer’s approach was his systematic use of recently developed tools such as homotopy, the underlying concept of the Poincaré group. In the following year, Hadamard generalised the theorem under discussion to an arbitrary finite dimension, but he employed different methods. Hans Freudenthal comments on the respective roles as follows: „Compared to Brouwer’s revolutionary methods, those of Hadamard were very traditional, but Hadamard’s participation in the birth of Brouwer’s ideas resembles that of a midwife more than that of a mere spectator.“
Brouwer’s approach yielded its fruits, and in 1910 he also found a proof that was valid for any finite dimension, as well as other key theorems such as the invariance of dimension. In the context of this work, Brouwer also generalized the Jordan curve theorem to arbitrary dimension and established the properties connected with the degree of a continuous mapping. This branch of mathematics, originally envisioned by Poincaré and developed by Brouwer, changed its name. In the 1930s, analysis situs became algebraic topology.
The theorem proved its worth in more than one way. During the 20th century numerous fixedpoint theorems were developed, and even a branch of mathematics called fixedpoint theory. Brouwer’s theorem is probably the most important. It is also among the foundational theorems on the topology of topological manifolds and is often used to prove other important results such as the Jordan curve theorem.
Besides the fixedpoint theorems for more or less contracting functions, there are many that have emerged directly or indirectly from the result under discussion. A continuous map from a closed ball of Euclidean space to its boundary cannot be the identity on the boundary. Similarly, the Borsuk–Ulam theorem says that a continuous map from the ndimensional sphere to R^{n} has a pair of antipodal points that are mapped to the same point. In the finitedimensional case, the Lefschetz fixedpoint theorem provided from 1926 a method for counting fixed points. In 1930, Brouwer’s fixedpoint theorem was generalized to Banach spaces. This generalization is known as Schauder’s fixedpoint theorem, a result generalized further by S. Kakutani to multivalued functions. One also meets the theorem and its variants outside topology. It can be used to prove the HartmanGrobman theorem, which describes the qualitative behaviour of certain differential equations near certain equilibria. Similarly, Brouwer’s theorem is used for the proof of the Central Limit Theorem. The theorem can also be found in existence proofs for the solutions of certain partial differential equations.
Other areas are also touched. In game theory, John Nash used the theorem to prove that in the game of Hex there is a winning strategy for white. In economics, P. Bich explains that certain generalizations of the theorem show that its use is helpful for certain classical problems in game theory and generally for equilibria (Hotelling’s law), financial equilibria and incomplete markets.
Brouwer’s celebrity is not exclusively due to his topological work. The proofs of his great topological theorems are not constructive, and Brouwer’s dissatisfaction with this is partly what led him to articulate the idea of constructivity. He became the originator and zealous defender of a way of formalising mathematics that is known as intuitionism, which at the time made a stand against set theory. The fixedpoint theorem is, as he originally stated it, false in intuitionism, and Brouwer disavowed it, proposing instead alternative versions to be constructively proven.
Brouwer’s original 1911 proof relied on the notion of the degree of a continuous mapping soccer uniform kits wholesale. Modern accounts of the proof can also be found in the literature.
Let
$$K
=
B
(
0
)
¯
{\displaystyle K={\overline {B(0)}}}
denote the closed unit ball in
$$R
n
{\displaystyle \mathbb {R} ^{n}}
centered at the origin. Suppose for simplicitly that
$$f
:
K
→
K
{\displaystyle f:K\to K}
is continuously differentiable. A regular value of
$$f
{\displaystyle f}
is a point
$$p
∈
B
(
0
)
{\displaystyle p\in B(0)}
such that the Jacobian of
$$f
{\displaystyle f}
is nonsingular at every point of the preimage of
$$p
{\displaystyle p}
. In particular, by the inverse function theorem, every point of the preimage of
$$f
{\displaystyle f}
lies in
$$B
(
0
)
{\displaystyle B(0)}
(the interior of
$$K
{\displaystyle K}
). The degree of
$$f
{\displaystyle f}
at a regular value
$$p
∈
B
(
0
)
{\displaystyle p\in B(0)}
is defined as the sum of the signs of the Jacobian determinant of
$$f
{\displaystyle f}
over the preimages of
$$p
{\displaystyle p}
under
$$f
{\displaystyle f}
:
The degree is, roughly speaking, the number of „sheets“ of the preimage f lying over a small open set around p, with sheets counted oppositely if they are oppositely oriented. This is thus a generalization of winding number to higher dimensions.
The degree satisfies the property of homotopy invariance: let
$$f
{\displaystyle f}
and
$$g
{\displaystyle g}
be two continuously differentiable functions, and
$$H
t
(
x
)
=
t
f
+
(
1
−
t
)
g
{\displaystyle H_{t}(x)=tf+(1t)g}
for
$$0
≤
t
≤
1
{\displaystyle 0\leq t\leq 1}
. Suppose that the point
$$p
{\displaystyle p}
is a regular value of
$$H
t
{\displaystyle H_{t}}
for all t. Then
$$deg
p
f
=
deg
p
g
{\displaystyle \deg _{p}f=\deg _{p}g}
.
If there is no fixed point of the boundary of
$$K
{\displaystyle K}
, then the function
is a homotopy from
$$g
(
x
)
=
x
−
f
(
x
)
{\displaystyle g(x)=xf(x)}
to the identity function. The identity function has degree one at every point. In particular, the identity function has degree one at the origin, so
$$g
{\displaystyle g}
also has degree one at the origin. As a consequence, the preimage
$$g
−
1
(
0
)
{\displaystyle g^{1}(0)}
is not empty. The elements of
$$g
−
1
(
0
)
{\displaystyle g^{1}(0)}
are precisely the fixed points of the original function f.
This requires some work to make fully general. The definition of degree must be extended to singular values of f, and then to continuous functions. The more modern advent of homology theory simplifies the construction of the degree, and so has become a standard proof in the literature.
The proof uses the observation that the boundary of D^{ n} is S^{ n − 1}, the (n − 1)sphere.
The argument proceeds by contradiction, supposing that a continuous function f : D^{ n} → D^{ n} has no fixed point, and then attempting to derive an inconsistency, which proves that the function must in fact have a fixed point. For each x in D^{ n}, there is only one straight line that passes through f(x) and x, because it must be the case that f(x) and x are distinct by hypothesis (recall that f having no fixed points means that f(x) ≠ x). Following this line from f(x) through x leads to a point on S^{ n − 1}, denoted by F(x). This defines a continuous function F : D^{ n} → S^{ n − 1}, which is a special type of continuous function known as a retraction: every point of the codomain (in this case S^{ n − 1}) is a fixed point of the function.
Intuitively it seems unlikely that there could be a retraction of D^{ n} onto S^{ n − 1}, and in the case n = 1 it is obviously impossible because S^{ 0} (i.e., the endpoints of the closed interval D^{ 1}) is not even connected. The case n = 2 is less obvious, but can be proven by using basic arguments involving the fundamental groups of the respective spaces: the retraction would induce an injective group homomorphism from the fundamental group of S^{ 1} to that of D^{ 2}, but the first group is isomorphic to Z while the latter group is trivial, so this is impossible. The case n = 2 can also be proven by contradiction based on a theorem about nonvanishing vector fields.
For n > 2, however, proving the impossibility of the retraction is more difficult. One way is to make use of homology groups: the homology H_{n − 1}(D^{ n}) is trivial, while H_{n − 1}(S^{ n − 1}) is infinite cyclic. This shows that the retraction is impossible, because again the retraction would induce an injective group homomorphism from the latter to the former group.
To prove that a map has fixed points, one can assume that it is smooth, because if a map has no fixed points then convolving it with a smooth function of sufficiently small support and integral equal to one produces a smooth function with no fixed points. As in the proof using homology, one is reduced to proving that there is no smooth retraction f from the ball B onto its boundary ∂B. If ω is a volume form on the boundary then by Stokes Theorem,
giving a contradiction.
More generally, this shows that there is no smooth retraction from any nonempty smooth orientable compact manifold onto its boundary. The proof using Stokes’s theorem is closely related to the proof using homology (or rather cohomology), because the form ω generates the de Rham cohomology group H^{n−1}(∂B) used in the cohomology proof.
The BFPT can be proved based on Sperner’s lemma. We now give an outline of the proof for the special case in which f is a function from the unit nsimplex to itself, i.e.:
Where:
For every point
$$P
∈
Δ
n
{\displaystyle P\in \Delta ^{n}}
, also
$$f
(
P
)
∈
Δ
n
{\displaystyle f(P)\in \Delta ^{n}}
. Hence the sum of their coordinates is equal:
Hence, by the pigeonhole principle, for every
$$P
∈
Δ
n
{\displaystyle P\in \Delta ^{n}}
there must be an index
$$j
∈
{
0
,
⋯
,
n
}
{\displaystyle j\in \{0,\cdots ,n\}}
such that the
$$j
{\displaystyle j}
th coordinate of
$$P
{\displaystyle P}
is weakly larger than the
$$j
{\displaystyle j}
th coordinate of its image under f:
Moreover, if
$$P
{\displaystyle P}
lies on a kdimensional subface of
$$Δ
n
{\displaystyle \Delta ^{n}}
, then by the same argument, the index
$$j
{\displaystyle j}
can be selected from among the (k+1) coordinates which are not zero on this subface.
We now use this fact to construct a Sperner coloring. For every triangulation of
$$Δ
n
{\displaystyle \Delta ^{n}}
, the color of every vertex
$$P
{\displaystyle P}
is an index
$$j
{\displaystyle j}
such that
$$f
(
P
)
j
≤
P
j
{\displaystyle f(P)_{j}\leq P_{j}}
.
By construction, this is a Sperner coloring. Hence, by Sperner’s lemma, there is an ndimensional simplex whose vertices are colored with the entire set of (n+1) available colors.
Because f is continuous, this simplex can be made arbitrarily small by choosing an arbitrarily fine triangulation. Hence, there must be a point
$$P
{\displaystyle P}
which satisfies the labeling condition in all coordinates, i.e.:
Because the sum of the coordinates of
$$P
{\displaystyle P}
and
$$f
(
P
)
{\displaystyle f(P)}
must be equal, all these inequalities must actually be equalities. But this means that:
I.e,
$$P
{\displaystyle P}
is a fixed point of
$$f
{\displaystyle f}
.
The proof can be extended to every object which is homeomorphic to
$$Δ
n
{\displaystyle \Delta ^{n}}
.
There is also a quick proof, by Morris Hirsch, based on the impossibility of a differentiable retraction. The indirect proof starts by noting that the map f can be approximated by a smooth map retaining the property of not fixing a point; this can be done by using the Weierstrass approximation theorem, for example. One then defines a retraction as above which must now be differentiable. Such a retraction must have a nonsingular value, by Sard’s theorem, which is also nonsingular for the restriction to the boundary (which is just the identity). Thus the inverse image would be a 1manifold with boundary. The boundary would have to contain at least two end points, both of which would have to lie on the boundary of the original ball—which is impossible in a retraction.
Kellogg, Li, and Yorke turned Hirsch’s proof into a constructive proof by observing that the retract is in fact defined everywhere except at the fixed points. For almost any point, q, on the boundary, (assuming it is not a fixed point) the one manifold with boundary mentioned above does exist and the only possibility is that it leads from q to a fixed point. It is an easy numerical task to follow such a path from q to the fixed point so the method is essentially constructive. Chow, MalletParet, and Yorke gave a conceptually similar pathfollowing version of the homotopy proof which extends to a wide variety of related problems.
A variation of the preceding proof does not employ the Sard’s theorem, and goes as follows. If r : B→∂B is a smooth retraction, one considers the smooth deformation g^{t}(x) := t r(x) + (1t)x, and the smooth function
Differentiating under the sign of integral it is not difficult to check that φ′(t)=0 for all t, so φ is a constant function, which is a contradiction because φ(0) is the ndimensional volume of the ball, while φ(1) is zero. The geometric idea is that φ(t) is the oriented area of g^{t}(B) (that is, the Lebesgue measure of the image of the ball via g^{t}, taking into account multiplicity and orientation), and should remain constant (as it is very clear in the onedimensional case). On the other hand, as the parameter t passes form 0 to 1 the map g^{t} transforms continuously from the identity map of the ball, to the retraction r, which is a contradiction since the oriented area of the identity coincides with the volume of the ball, while the oriented area of r is necessarily 0, as its image is the boundary of the ball, a set of null measure.
A quite different proof given by David Gale is based on the game of Hex. The basic theorem about Hex is that no game can end in a draw. This is equivalent to the Brouwer fixedpoint theorem for dimension 2. By considering ndimensional versions of Hex, one can prove in general that Brouwer’s theorem is equivalent to the determinacy theorem for Hex.
The Lefschetz fixedpoint theorem says that if a continuous map f from a finite simplicial complex B to itself has only isolated fixed points, then the number of fixed points counted with multiplicities (which may be negative) is equal to the Lefschetz number
and in particular if the Lefschetz number is nonzero then f must have a fixed point. If B is a ball (or more generally is contractible) then the Lefschetz number is one because the only nonzero homology group is :
$$H
0
(
B
)
{\displaystyle H_{0}(B)}
and f acts as the identity on this group, so f has a fixed point.
In reverse mathematics, Brouwer’s theorem can be proved in the system WKL0, and conversely over the base system RCA0 Brouwer’s theorem for a square implies the weak König’s lemma, so this gives a precise description of the strength of Brouwer’s theorem.
The Brouwer fixedpoint theorem forms the starting point of a number of more general fixedpoint theorems.
The straightforward generalization to infinite dimensions, i.e. using the unit ball of an arbitrary Hilbert space instead of Euclidean space, is not true. The main problem here is that the unit balls of infinitedimensional Hilbert spaces are not compact. For example, in the Hilbert space ℓ2 of squaresummable real (or complex) sequences, consider the map f : ℓ^{2} → ℓ^{2} which sends a sequence (x_{n}) from the closed unit ball of ℓ^{2} to the sequence (y_{n}) defined by
It is not difficult to check that this map is continuous, has its image in the unit sphere of ℓ^{ 2}, but does not have a fixed point.
The generalizations of the Brouwer fixedpoint theorem to infinite dimensional spaces therefore all include a compactness assumption of some sort, and in addition also often an assumption of convexity. See fixedpoint theorems in infinitedimensional spaces for a discussion of these theorems.
There is also finitedimensional generalization to a larger class of spaces: If
$$X
{\displaystyle X}
is a product of finitely many chainable continua, then every continuous function
$$f
:
X
→
X
{\displaystyle f:X\rightarrow X}
has a fixed point, where a chainable continuum is a (usually but in this case not necessarily metric) compact Hausdorff space of which every open cover has a finite open refinement
$${
U
1
,
…
,
U
m
}
{\displaystyle \{U_{1},\ldots ,U_{m}\}}
, such that
$$U
i
∩
U
j
≠
∅
{\displaystyle U_{i}\cap U_{j}\neq \emptyset }
if and only if
$$
i
−
j

≤
1
{\displaystyle ij\leq 1}