Foliation Explained

In mathematics (differential geometry), a foliation is an equivalence relation on an n-manifold, the equivalence classes being connected, injectively immersed submanifolds, all of the same dimension p, modeled on the decomposition of the real coordinate space Rⁿ into the cosets x + R^p of the standardly embedded subspace R^p. The equivalence classes are called the leaves of the foliation. If the manifold and/or the submanifolds are required to have a piecewise-linear, differentiable (of class C^r), or analytic structure then one defines piecewise-linear, differentiable, or analytic foliations, respectively. In the most important case of differentiable foliation of class C^r it is usually understood that r ≥ 1 (otherwise, C⁰ is a topological foliation). The number p (the dimension of the leaves) is called the dimension of the foliation and is called its codimension.

In some papers on general relativity by mathematical physicists, the term foliation (or slicing) is used to describe a situation where the relevant Lorentz manifold (a (p+1)-dimensional spacetime) has been decomposed into hypersurfaces of dimension p, specified as the level sets of a real-valued smooth function (scalar field) whose gradient is everywhere non-zero; this smooth function is moreover usually assumed to be a time function, meaning that its gradient is everywhere time-like, so that its level-sets are all space-like hypersurfaces. In deference to standard mathematical terminology, these hypersurface are often called the leaves (or sometimes slices) of the foliation. Note that while this situation does constitute a codimension-1 foliation in the standard mathematical sense, examples of this type are actually globally trivial; while the leaves of a (mathematical) codimension-1 foliation are always locally the level sets of a function, they generally cannot be expressed this way globally, as a leaf may pass through a local-trivializing chart infinitely many times, and the holonomy around a leaf may also obstruct the existence of a globally-consistent defining functions for the leaves. For example, while the 3-sphere has a famous codimension-1 foliation discovered by Reeb, a codimension-1 foliation of a closed manifold cannot be given by the level sets of a smooth function, since a smooth function on a closed manifold necessarily has critical points at its maxima and minima.

Foliated charts and atlases

In order to give a more precise definition of foliation, it is necessary to define some auxiliary elements.A rectangular neighborhood in Rⁿ is an open subset of the form B = J₁ × ⋅⋅⋅ × J_n, where J_i is a (possibly unbounded) relatively open interval in the ith coordinate axis. If J₁ is of the form (a,0], it is said that B has boundary

\partial B = \left \{ \left( 0 ,x ^{2,} \ldots, x ^{n} \right ) \in B \right \}. In the following definition, coordinate charts are considered that have values in R^p × R^q, allowing the possibility of manifolds with boundary and (convex) corners.

A foliated chart on the n-manifold M of codimension q is a pair (U,φ), where U ⊆ M is open and

\varphi: U \to B _{\tau} x B _{\pitchfork} is a diffeomorphism, B _{\pitchfork} being a rectangular neighborhood in R^q and B _{\tau} a rectangular neighborhood in R^p. The set P_y = φ⁻¹(B_τ ×), where y \in B _{\pitchfork} , is called a plaque of this foliated chart. For each x ∈ B_τ, the set S_x = φ⁻¹(× B _{\pitchfork} ) is called a transversal of the foliated chart. The set ∂_τU = φ⁻¹(B_τ × (∂B _{\pitchfork} )) is called the tangential boundary of U and \partial _{\pitchfork} U = φ⁻¹((∂B_τ) × B _{\pitchfork} ) is called the transverse boundary of U.

The foliated chart is the basic model for all foliations, the plaques being the leaves. The notation B_τ is read as "B-tangential" and

B _{\pitchfork} as "B-transverse". There are also various possibilities. If both B _{\pitchfork} and B_τ have empty boundary, the foliated chart models codimension-q foliations of n-manifolds without boundary. If one, but not both of these rectangular neighborhoods has boundary, the foliated chart models the various possibilities for foliations of n-manifolds with boundary and without corners. Specifically, if ∂B _{\pitchfork} ≠ ∅ = ∂B_τ, then ∂U = ∂_τU is a union of plaques and the foliation by plaques is tangent to the boundary. If ∂B_τ ≠ ∅ = ∂B _{\pitchfork} , then ∂U = \partial _{\pitchfork} U is a union of transversals and the foliation is transverse to the boundary. Finally, if ∂B _{\pitchfork} ≠ ∅ ≠ ∂B_τ, this is a model of a foliated manifold with a corner separating the tangential boundary from the transverse boundary.

A foliated atlas of codimension q and class C^r (0 ≤ r ≤ ∞) on the n-manifold M is a C^r-atlas

l{U} = \{(U _{\alpha} , \varphi _{\alpha} )\mid \alpha \in A\} of foliated charts of codimension q which are coherently foliated in the sense that, whenever P and Q are plaques in distinct charts of l{U} , then P ∩ Q is open both in P and Q.

A useful way to reformulate the notion of coherently foliated charts is to write for w ∈ U_α ∩ U_β

\varphi _{\alpha} (w) = \left ( x _{\alpha} (w), y _{\alpha} (w) \right ) \in B ^{\alpha} _{\tau} x B ^{\alpha} _{\pitchfork} , \varphi _{\beta} (w) = \left ( x _{\beta} (w), y _{\beta} (w) \right ) \in B ^{\beta} _{\tau} x B ^{\beta} _{\pitchfork} . The notation (U_α,φ_α) is often written (U_α,x_α,y_α), with x _{\alpha} = \left (x ^{1 ,} _{\alpha} ... ,x ^{p} _{\alpha} \right ), y _{\alpha} = \left (y ^{1 ,} _{\alpha} ... ,y ^{q} _{\alpha} \right ).

On φ_β(U_α ∩ U_β) the coordinates formula can be changed as

g _{\alpha} \left ( x _{\beta} ,y _{\beta} \right ) = \varphi _{\alpha} \circ \varphi ^{-1} _{\beta} \left ( x _{\beta} , y _{\beta} \right ) = \left ( x _{\alpha} \left ( x _{\beta} , y _{\beta} \right ), y _{\alpha} \left ( x _{\beta} , y _{\beta} \right ) \right ). The condition that (U_α,x_α,y_α) and (U_β,x_β,y_β) be coherently foliated means that, if P ⊂ U_α is a plaque, the connected components of P ∩ U_β lie in (possibly distinct) plaques of U_β. Equivalently, since the plaques of U_α and U_β are level sets of the transverse coordinates y_α and y_β, respectively, each point z ∈ U_α ∩ U_β has a neighborhood in which the formulay _{\alpha} = y _{\alpha(x} _{\beta,} y _{\beta)} = y _{\alpha(y} _{\beta)} is independent of x_β.

The main use of foliated atlases is to link their overlapping plaques to form the leaves of a foliation. For this and other purposes, the general definition of foliated atlas above is a bit clumsy. One problem is that a plaque of (U_α,φ_α) can meet multiple plaques of (U_β,φ_β). It can even happen that a plaque of one chart meets infinitely many plaques of another chart. However, no generality is lost in assuming the situation to be much more regular as shown below.

Two foliated atlases

l{U} and l{V} on M of the same codimension and smoothness class C^r are coherent \left ( l{U} \thickapprox l{V} \right ) if l{U} \cup l{V} is a foliated C^r-atlas. Coherence of foliated atlases is an equivalence relation.Plaques and transversals defined above on open sets are also open. But one can speak also of closed plaques and transversals. Namely, if (U,φ) and (W,ψ) are foliated charts such that \overline{U} (the closure of U) is a subset of W and φ = ψ|U then, if \varphi(U) = B _{\tau} x B _{\pitchfork} , it can be seen that \psi|\overline{U} , written \overline{\varphi} , carries \overline{U} diffeomorphically onto \overline{B} _{\tau} x \overline{B} _{\pitchfork} .

A foliated atlas is said to be regular if

1. for each α ∈ A,

\overline{U} _{\alpha} is a compact subset of a foliated chart (W_α,ψ_α) and φ_α = ψ_α|U_α;

1. the cover is locally finite;
2. if (U_α,φ_α) and (U_β,φ_β) are elements of the foliated atlas, then the interior of each closed plaque P ⊂

\overline{U} _{\alpha} meets at most one plaque in \overline{U} _{\beta} .

By property (1), the coordinates x_α and y_α extend to coordinates

\overline{x} _{\alpha} and \overline{y} _{\alpha} on \overline{U} _{\alpha} and one writes \overline{\varphi} _{\alpha} = \left (\overline{x} _{\alpha} ,\overline{y} _{\alpha} \right ). Property (3) is equivalent to requiring that, if U_α ∩ U_β ≠ ∅, the transverse coordinate changes \overline{y} _{\alpha} = \overline{y} _{\alpha} \left ( \overline{x} _{\beta} , \overline{y} _{\beta} \right ) be independent of \overline{x} _{\beta} . That is\overline{g} _{\alpha} = \overline{\varphi} _{\alpha} \circ \overline{\varphi} ^{-1} _{\beta} : \overline{\varphi} _{\beta} \left ( \overline{U} _{\alpha} \cap \overline{U} _{\beta} \right ) → \overline{\varphi} _{\alpha} \left ( \overline{U} _{\alpha} \cap \overline{U} _{\beta} \right ) has the formula \overline{g} _{\alpha} \left ( \overline{x} _{\beta} , \overline{y} _{\beta} \right ) = \left ( \overline{x} _{\alpha} \left ( \overline{x} _{\beta} , \overline{y} _{\beta} \right ), \overline{y} _{\alpha} \left ( \overline{y} _{\beta} \right ) \right ). Similar assertions hold also for open charts (without the overlines). The transverse coordinate map y_α can be viewed as a submersiony _{\alpha} : U _{\alpha} → R ^{q} and the formulas y_α = y_α(y_β) can be viewed as diffeomorphisms\gamma _{\alpha} : y _{\beta} \left ( U _{\alpha} \cap U _{\beta} \right ) → y _{\alpha} \left ( U _{\alpha} \cap U _{\beta} \right ). These satisfy the cocycle conditions. That is, on y_δ(U_α ∩ U_β ∩ U_δ),\gamma _{\alpha} = \gamma _{\alpha} \circ \gamma _{\beta} and, in particular,\gamma _{\alpha} \equiv y _{\alpha} \left ( U _{\alpha} \right ), \gamma _{\alpha} = \gamma ^{-1} _{\beta \alpha} . Using the above definitions for coherence and regularity it can be proven that every foliated atlas has a coherent refinement that is regular.

Foliation definitions

Several alternative definitions of foliation exist depending on the way through which the foliation is achieved. The most common way to achieve a foliation is through decomposition reaching to the followingDefinition. A p-dimensional, class C^r foliation of an n-dimensional manifold M is a decomposition of M into a union of disjoint connected submanifolds _α∈A, called the leaves of the foliation, with the following property: Every point in M has a neighborhood U and a system of local, class C^r coordinates x=(x¹, ⋅⋅⋅, xⁿ) : U→Rⁿ such that for each leaf L_α, the components of U ∩ L_α are described by the equations x^p+1=constant, ⋅⋅⋅, xⁿ=constant. A foliation is denoted by

l{F} =_α∈A.

The notion of leaves allows for an intuitive way of thinking about a foliation. For a slightly more geometrical definition, -dimensional foliation

l{F} of an -manifold may be thought of as simply a collection of pairwise-disjoint, connected, immersed -dimensional submanifolds (the leaves of the foliation) of, such that for every point in, there is a chart (U,\varphi) with homeomorphic to containing such that every leaf,, meets in either the empty set or a countable collection of subspaces whose images under \varphi in \varphi (M _{a} \cap U) are -dimensional affine subspaces whose first coordinates are constant.

Locally, every foliation is a submersion allowing the following

Definition. Let M and Q be manifolds of dimension n and q≤n respectively, and let f : M→Q be a submersion, that is, suppose that the rank of the function differential (the Jacobian) is q. It follows from the Implicit Function Theorem that ƒ induces a codimension-q foliation on M where the leaves are defined to be the components of f⁻¹(x) for x ∈ Q.

This definition describes a dimension- foliation

l{F} of an -dimensional manifold that is a covered by charts together with maps\varphi _{i:U} _{i} \to R ^{n}

such that for overlapping pairs the transition functions defined by

\varphi _{ij} = \varphi _{j} \varphi ^{-1} _{i}

take the form

\varphi _{ij} (x,y) = (\varphi ^{2 (x,y))} _{ij}

where denotes the first = coordinates, and denotes the last co-ordinates. That is,

\begin{align} \varphi ^{1 :{}} _{ij} & R ^{q\toR} ^{q} \\ \varphi ^{2 :{}} _{ij} & R ^{n\toR} ^{p \end{align}} The splitting of the transition functions φ_ij into \varphi ^{1 (x)} _{ij} and \varphi ^{2 (x,y)} _{ij} as a part of the submersion is completely analogous to the splitting of \overline{g} _{\alpha} into \overline{y} _{\alpha} \left ( \overline{y} _{\beta} \right ) and \overline{x} _{\alpha} \left ( \overline{x} _{\beta} , \overline{y} _{\beta} \right ) as a part of the definition of a regular foliated atlas. This makes possible another definition of foliations in terms of regular foliated atlases. To this end, one has to prove first that every regular foliated atlas of codimension q is associated to a unique foliation l{F} of codimension q.As shown in the proof, the leaves of the foliation are equivalence classes of plaque chains of length ≤ p which are also topologically immersed Hausdorff -dimensional submanifolds. Next, it is shown that the equivalence relation of plaques on a leaf is expressed in equivalence of coherent foliated atlases in respect to their association with a foliation. More specifically, if l{U} and l{V} are foliated atlases on M and if l{U} is associated to a foliation l{F} then l{U} and l{V} are coherent if and only if l{V} is also associated to l{F} .It is now obvious that the correspondence between foliations on M and their associated foliated atlases induces a one-to-one correspondence between the set of foliations on M and the set of coherence classes of foliated atlases or, in other words, a foliation l{F} of codimension q and class C^r on M is a coherence class of foliated atlases of codimension q and class C^r on M. By Zorn's lemma, it is obvious that every coherence class of foliated atlases contains a unique maximal foliated atlas. Thus,

Definition. A foliation of codimension q and class C^r on M is a maximal foliated C^r-atlas of codimension q on M.

In practice, a relatively small foliated atlas is generally used to represent a foliation. Usually, it is also required this atlas to be regular.

In the chart, the stripes match up with the stripes on other charts . These submanifolds piece together from chart to chart to form maximal connected injectively immersed submanifolds called the leaves of the foliation.

If one shrinks the chart it can be written as, where, is homeomorphic to the plaques, and the points of parametrize the plaques in . If one picks in, then is a submanifold of that intersects every plaque exactly once. This is called a local transversal section of the foliation. Note that due to monodromy global transversal sections of the foliation might not exist.

The case r = 0 is rather special. Those C⁰ foliations that arise in practice are usually "smooth-leaved". More precisely, they are of class C^r,0, in the following sense.

Definition. A foliation

l{F} is of class C^r,k, r > k ≥ 0, if the corresponding coherence class of foliated atlases contains a regular foliated atlas _α∈A such that the change of coordinate formulag _{\alpha} (x _{\beta,} y _{\beta)} = ( x _{\alpha(} x _{\beta,} y _{\beta),} y _{\alpha} ( y _{\beta)).} is of class C^k, but x_α is of class C^r in the coordinates x_β and its mixed x_β partials of orders ≤ r are C^k in the coordinates (x_β,y_β).

The above definition suggests the more general concept of a foliated space or abstract lamination. One relaxes the condition that the transversals be open, relatively compact subsets of R^q, allowing the transverse coordinates y_α to take their values in some more general topological space Z. The plaques are still open, relatively compact subsets of R^p, the change of transverse coordinate formula y_α(y_β) is continuous and x_α(x_β,y_β) is of class C^r in the coordinates x_β and its mixed x_β partials of orders ≤ r are continuous in the coordinates (x_β,y_β). One usually requires M and Z to be locally compact, second countable and metrizable. This may seem like a rather wild generalization, but there are contexts in which it is useful.

Holonomy

Let (M,

l{F} ) be a foliated manifold. If L is a leaf of l{F} and s is a path in L, one is interested in the behavior of the foliation in a neighborhood of s in M. Intuitively, an inhabitant of the leaf walks along the path s, keeping an eye on all of the nearby leaves. As they (hereafter denoted by s(t)) proceed, some of these leaves may "peel away", getting out of visual range, others may suddenly come into range and approach L asymptotically, others may follow along in a more or less parallel fashion or wind around L laterally, etc. If s is a loop, then s(t) repeatedly returns to the same point s(t₀) as t goes to infinity and each time more and more leaves may have spiraled into view or out of view, etc. This behavior, when appropriately formalized, is called the holonomy of the foliation.

Holonomy is implemented on foliated manifolds in various specific ways: the total holonomy group of foliated bundles, the holonomy pseudogroup of general foliated manifolds, the germinal holonomy groupoid of general foliated manifolds, the germinal holonomy group of a leaf, and the infinitesimal holonomy group of a leaf.

Foliated bundles

The easiest case of holonomy to understand is the total holonomy of a foliated bundle. This is a generalization of the notion of a Poincaré map.The term "first return (recurrence) map" comes from the theory of dynamical systems. Let Φ_t be a nonsingular C^r flow (r ≥ 1) on the compact n-manifold M. In applications, one can imagine that M is a cyclotron or some closed loop with fluid flow. If M has a boundary, the flow is assumed to be tangent to the boundary. The flow generates a 1-dimensional foliation

l{F} . If one remembers the positive direction of flow, but otherwise forgets the parametrization (shape of trajectory, velocity, etc.), the underlying foliation l{F} is said to be oriented. Suppose that the flow admits a global cross section N. That is, N is a compact, properly embedded, C^r submanifold of M of dimension n – 1, the foliation l{F} is transverse to N, and every flow line meets N. Because the dimensions of N and of the leaves are complementary, the transversality condition is thatT _{y} (M) = T _{y(l{F}) ⊕} T _{y(N)} foreach y\in N.

Let y ∈ N and consider the ω-limit set ω(y) of all accumulation points in M of all sequences

\left \{\Phi _{t _{k}} (y)\right\} ^{inf ty} _{k =1} , where t_k goes to infinity. It can be shown that ω(y) is compact, nonempty, and a union of flow lines. If z = \lim _{k} \Phi _{t _{k}} \in \omega(y), there is a value t* ∈ R such that Φ_t*(z) ∈ N and it follows that\lim _{k} \Phi _{t _{k} + t ^{\ast}} (y) = \Phi _{t ^{\ast}} (z) \in N. Since N is compact and l{F} is transverse to N, it follows that the set is a monotonically increasing sequence \{\tau _{k(y)\}} ^{inf ty} ^{k =1} that diverges to infinity.

As y ∈ N varies, let τ(y) = τ₁(y), defining in this way a positive function τ ∈ C^r(N) (the first return time) such that, for arbitrary y ∈ N, Φ_t(y) ∉ N, 0 < t < τ(y), and Φ_τ(y)(y) ∈ N.

Define f : N → N by the formula f(y) = Φ_τ(y)(y). This is a C^r map. If the flow is reversed, exactly the same construction provides the inverse f⁻¹; so f ∈ Diff^r(N). This diffeomorphism is the first return map and τ is called the first return time. While the first return time depends on the parametrization of the flow, it should be evident that f depends only on the oriented foliation

l{F} . It is possible to reparametrize the flow Φ_t, keeping it nonsingular, of class C^r, and not reversing its direction, so that τ ≡ 1.

The assumption that there is a cross section N to the flow is very restrictive, implying that M is the total space of a fiber bundle over S¹. Indeed, on R × N, define ~_f to be the equivalence relation generated by

(t,y) \sim _{f} (t -1 ,f(y)). Equivalently, this is the orbit equivalence for the action of the additive group Z on R × N defined byk ⋅ (t,y) = (t - k,f ^{k(y)} ), for each k ∈ Z and for each (t,y) ∈ R × N. The mapping cylinder of f is defined to be the C^r manifoldM _{f} = ( R x N)/{\sim _{f}.} By the definition of the first return map f and the assumption that the first return time is τ ≡ 1, it is immediate that the map\Phi : R x N → M. defined by the flow, induces a canonical C^r diffeomorphism \varphi : M _{f} → M. If we make the identification M_f = M, then the projection of R × N onto R induces a C^r map\pi : M → R / Z = S ^{1} that makes M into the total space of a fiber bundle over the circle. This is just the projection of S¹ × D² onto S¹. The foliation l{F} is transverse to the fibers of this bundle and the bundle projection , restricted to each leaf L, is a covering map : L → S¹. This is called a foliated bundle.

Take as basepoint x₀ ∈ S¹ the equivalence class 0 + Z; so π⁻¹(x₀) is the original cross section N. For each loop s on S¹, based at x₀, the homotopy class [''s''] ∈ π₁(S¹,x₀) is uniquely characterized by deg s ∈ Z. The loop s lifts to a path in each flow line and it should be clear that the lift s_y that starts at y ∈ N ends at f^k(y) ∈ N, where k = deg s. The diffeomorphism f^k ∈ Diff^r(N) is also denoted by h_s and is called the total holonomy of the loop s. Since this depends only on [''s''], this is a definition of a homomorphism

h : \pi ^{1 ,x} _{0 )} → \operatorname{Diff} ^{r} (N), called the total holonomy homomorphism for the foliated bundle.

Using fiber bundles in a more direct manner, let (M,

l{F} ) be a foliated n-manifold of codimension q. Let : M → B be a fiber bundle with q-dimensional fiber F and connected base space B. Assume that all of these structures are of class C^r, 0 ≤ r ≤ ∞, with the condition that, if r = 0, B supports a C¹ structure. Since every maximal C¹ atlas on B contains a C^∞ subatlas, no generality is lost in assuming that B is as smooth as desired. Finally, for each x ∈ B, assume that there is a connected, open neighborhood U ⊆ B of x and a local trivialization\begin{matrix} \pi ^{-1} (U) & \xrightarrow{\varphi} & U x {F} \\ \scriptstyle{\pi} \ downarrow & { } & \ downarrow{\scriptstyle{p}} \\ U & \xrightarrow{ id } & U\endwhere φ is a C^r diffeomorphism (a homeomorphism, if r = 0) that carries $\backslash mathcal\; \backslash mid\; \backslash pi^(U)$ to the product foliation _y ∈ F. Here, $\backslash mathcal\; \backslash mid\; \backslash pi^(U)$ is the foliation with leaves the connected components of L ∩ π⁻¹(U), where L ranges over the leaves of l{F} . This is the general definition of the term "foliated bundle" (M,l{F} ,π) of class C^r.l{F} is transverse to the fibers of π (it is said that l{F} is transverse to the fibration) and that the restriction of π to each leaf L of l{F} is a covering map π : L → B. In particular, each fiber F_x = ⁻¹(x) meets every leaf of l{F} . The fiber is a cross section of l{F} in complete analogy with the notion of a cross section of a flow.

The foliation

l{F} being transverse to the fibers does not, of itself, guarantee that the leaves are covering spaces of B. A simple version of the problem is a foliation of R², transverse to the fibration\pi : R ^{2} → R , \pi(x,y) = x, but with infinitely many leaves missing the y-axis. In the respective figure, it is intended that the "arrowed" leaves, and all above them, are asymptotic to the axis x = 0. One calls such a foliation incomplete relative to the fibration, meaning that some of the leaves "run off to infinity" as the parameter x ∈ B approaches some x₀ ∈ B. More precisely, there may be a leaf L and a continuous path s : [0,''a'') → ''L'' such that lim_{''t''→''a''−}π(''s''(''t'')) = ''x''₀ ∈ ''B'', but lim_{''t''→''a''−}''s''(''t'') does not exist in the manifold topology of ''L''. This is analogous to the case of incomplete flows, where some flow lines "go to infinity" in finite time. Although such a leaf ''L'' may elsewhere meet π⁻¹(''x''₀), it cannot evenly cover a neighborhood of ''x''₀, hence cannot be a covering space of ''B'' under {{pi}}. When ''F'' is compact, however, it is true that transversality of <math>\mathcal{F}</math> to the fibration does guarantee completeness, hence that <math display="inline">(M,\mathcal{F},\pi)</math> is a foliated bundle. There is an atlas <math>\mathcal{U}</math> = {''U''_α,''x''_α}_α∈A on ''B'', consisting of open, connected coordinate charts, together with trivializations ''φ''_''α'' : ''π''⁻¹(''U''_''α'') → ''U''_''α'' × ''F'' that carry <math>\mathcal{F}</math>|π⁻¹(''U''_''α'') to the product foliation. Set ''W''_''α'' = ''π''⁻¹(''U''_''α'') and write ''φ''_''α'' = (''x''_''α'',''y''_''α'') where (by abuse of notation) ''x''_α represents ''x''_''α'' ∘ ''π'' and ''y''_''α'' : ''π''⁻¹(''U''_α) → ''F'' is the submersion obtained by composing ''φ''_α with the canonical projection ''U''_α × ''F'' → ''F''. The atlas <math>\mathcal{W}</math> = {''W''_''α'',''x''_''α'',''y''_''α''}_{''α''∈''A''} plays a role analogous to that of a foliated atlas. The plaques of ''W''_''α'' are the level sets of ''y''_''α'' and this family of plaques is identical to ''F'' via ''y''_''α''. Since ''B'' is assumed to support a ''C''^∞ structure, according to the [[Whitehead theorem]] one can fix a Riemannian metric on B and choose the atlas l{U} to be geodesically convex. Thus, U_α ∩ U_β is always connected. If this intersection is nonempty, each plaque of W_α meets exactly one plaque of W_β. Then define a holonomy cocycle \gamma = \left \{ \gamma _{\alpha} \right \} _{\alpha,\beta} by setting\gamma _{\alpha} = y _{\alpha} \circ y ^{-1} _{\beta} : F → F.

Examples

Flat space

Consider an -dimensional space, foliated as a product by subspaces consisting of points whose first coordinates are constant. This can be covered with a single chart. The statement is essentially that with the leaves or plaques being enumerated by . The analogy is seen directly in three dimensions, by taking and : the 2-dimensional leaves of a book are enumerated by a (1-dimensional) page number.

Bundles

A rather trivial example of foliations are products, foliated by the leaves . (Another foliation of is given by .)

A more general class are flat -bundles with for a manifold . Given a representation, the flat -bundle with monodromy is given by

M = \left(\widetilde{B} x F\right)/\pi _{1B} , where acts on the universal cover \widetilde{B} by deck transformations and on by means of the representation .

Flat bundles fit into the framework of fiber bundles. A map between manifolds is a fiber bundle if there is a manifold F such that each has an open neighborhood such that there is a homeomorphism

\varphi:\pi ^{-1} (U)\to U x F with \pi = p _{1} \varphi , with projection to the first factor. The fiber bundle yields a foliation by fibers F ^{-1} _{b: = \pi} (\{b\}), b\in B . Its space of leaves L is homeomorphic to, in particular L is a Hausdorff manifold.

Coverings

If is a covering map between manifolds, and is a foliation on, then it pulls back to a foliation on . More generally, if the map is merely a branched covering, where the branch locus is transverse to the foliation, then the foliation can be pulled back.

Submersions

If is a submersion of manifolds, it follows from the inverse function theorem that the connected components of the fibers of the submersion define a codimension foliation of . Fiber bundles are an example of this type.

An example of a submersion, which is not a fiber bundle, is given by

\begin{cases} f:[ -1 , 1 ] x R \to R \\f(x,y) = (x ^{2-1)} e ^{y\end{cases}}

This submersion yields a foliation of which is invariant under the -actions given by

z(x, y) = (x,y + n ), or z(x, y) = \left(( -1 ) ^{nx,} y\right) for and . The induced foliations of are called the 2-dimensional Reeb foliation (of the annulus) resp. the 2-dimensional nonorientable Reeb foliation (of the Möbius band). Their leaf spaces are not Hausdorff.

Reeb foliations

Define a submersion

\begin{cases} f:D ^{n} x R \to R \ f(r,\theta,t): = (r ^{2-1)e} ^{t\end{cases}}

where are cylindrical coordinates on the -dimensional disk . This submersion yields a foliation of which is invariant under the -actions given by

z(x,y) = (x,y + z)

for . The induced foliation of is called the -dimensional Reeb foliation. Its leaf space is not Hausdorff.

For, this gives a foliation of the solid torus which can be used to define the Reeb foliation of the 3-sphere by gluing two solid tori along their boundary. Foliations of odd-dimensional spheres are also explicitly known.

Lie groups

If is a Lie group, and is a Lie subgroup, then is foliated by cosets of . When is closed in, the quotient space / is a smooth (Hausdorff) manifold turning into a fiber bundle with fiber and base /. This fiber bundle is actually principal, with structure group .

Lie group actions

Let be a Lie group acting smoothly on a manifold . If the action is a locally free action or free action, then the orbits of define a foliation of .

Linear and Kronecker foliations

If

\tilde{X} is a nonsingular (i.e., nowhere zero) vector field, then the local flow defined by \tilde{X} patches together to define a foliation of dimension 1. Indeed, given an arbitrary point x ∈ M, the fact that \tilde{X} is nonsingular allows one to find a coordinate neighborhood (U,x¹,...,xⁿ) about x such that- \varepsilon < x ^{i} < \varepsilon, 1 \le i \le n, and(\partial)/(\partial x ^{1}) = \tilde{X}\mid U. Geometrically, the flow lines of \tilde{X} \mid U are just the level setsx ^{i} = c ^{i,} 2 \le i \le n, where all |c ^{i|} < \varepsilon. Since by convention manifolds are second countable, leaf anomalies like the "long line" are precluded by the second countability of M itself. The difficulty can be sidestepped by requiring that \tilde{X} be a complete field (e.g., that M be compact), hence that each leaf be a flow line.An important class of 1-dimensional foliations on the torus T² are derived from projecting constant vector fields on T². A constant vector field\tilde{X} \equiv \begin{bmatrix}a \ b \end{bmatrix} on R² is invariant by all translations in R², hence passes to a well-defined vector field X when projected on the torus . It is assumed that a ≠ 0. The foliation l{\tilde{F}} on R² produced by \tilde{X} has as leaves the parallel straight lines of slope θ = b/a. This foliation is also invariant under translations and passes to the foliation l{F} on T² produced by X.

Each leaf of

l{\tilde{F}} is of the form\tilde{L} = \{(x _{0} + ta,y _{0} + tb)\} _{t} . If the slope is rational then all leaves are closed curves homeomorphic to the circle. In this case, one can take a,b ∈ Z. For fixed t ∈ R, the points of \tilde{L} corresponding to values of t ∈ t₀ + Z all project to the same point of T²; so the corresponding leaf L of l{F} is an embedded circle in T². Since L is arbitrary, l{F} is a foliation of T² by circles. It follows rather easily that this foliation is actually a fiber bundle π : T² → S¹. This is known as a linear foliation.

When the slope θ = b/a is irrational, the leaves are noncompact, homeomorphic to the non-compactified real line, and dense in the torus (cf Irrational rotation). The trajectory of each point (x₀,y₀) never returns to the same point, but generates an "everywhere dense" winding about the torus, i.e. approaches arbitrarily close to any given point. Thus the closure to the trajectory is the entire two-dimensional torus. This case is named Kronecker foliation, after Leopold Kronecker and his

Kronecker's Density Theorem. If the real number θ is distinct from each rational multiple of π, then the set is dense in the unit circle.

A similar construction using a foliation of by parallel lines yields a 1-dimensional foliation of the -torus associated with the linear flow on the torus.

Suspension foliations

A flat bundle has not only its foliation by fibres but also a foliation transverse to the fibers, whose leaves are

L _{f:=} \left\{p\left(\tilde{b},f\right): \tilde{b}\in\widetilde{B}\right\}, for f\in F, where p:\widetilde{B} x F\to M is the canonical projection. This foliation is called the suspension of the representation .

In particular, if and

\varphi:F\to F is a homeomorphism of, then the suspension foliation of \varphi is defined to be the suspension foliation of the representation given by . Its space of leaves is, where whenever for some .

The simplest example of foliation by suspension is a manifold X of dimension q. Let f : X → X be a bijection. One defines the suspension M = S¹ ×_f X as the quotient of [0,1] × X by the equivalence relation (1,x) ~ (0,f(x)).

M = S¹ ×_f X = [0,1] × XThen automatically M carries two foliations:

l{F} ₂ consisting of sets of the form F_2,t = and l{F} ₁ consisting of sets of the form F_2,x0 =, where the orbit O_x0 is defined as

O_x0 =,where the exponent refers to the number of times the function f is composed with itself. Note that O_x0 = O_f(x0) = O_f⁻²(x0), etc., so the same is true for F_1,x0. Understanding the foliation

l{F} ₁ is equivalent to understanding the dynamics of the map f. If the manifold X is already foliated, one can use the construction to increase the codimension of the foliation, as long as f maps leaves to leaves.

The Kronecker foliations of the 2-torus are the suspension foliations of the rotations by angle More specifically, if Σ = Σ₂ is the two-holed torus with C¹,C² ∈ Σ the two embedded circles let

l{F} be the product foliation of the 3-manifold M = Σ × S¹ with leaves Σ ×, y ∈ S¹. Note that N_i = C_i × S¹ is an embedded torus and that l{F} is transverse to N_i, i = 1,2. Let Diff₊(S¹) denote the group of orientation-preserving diffeomorphisms of S¹ and choose f₁,f₂ ∈ Diff₊(S¹). Cut M apart along N₁ and N₂, letting N ^{+} _{i} and N ^{-} _{i} denote the resulting copies of N_i, i = 1,2. At this point one has a manifold M = Σ' × S₁ with four boundary components \left \{N ^{\pm} _{i} \right \} _{i=1,2} . The foliation l{F} has passed to a foliation l{F ^{\prime} } transverse to the boundary ∂M' , each leaf of which is of the form Σ' ×, y ∈ S¹.

This leaf meets ∂M' in four circles

C ^{\pm} _{i} x \{y\} \subset N ^{\pm} _{i} . If z ∈ C_i, the corresponding points in C ^{\pm} _{i} are denoted by z^± and N ^{-} _{i} is "reglued" to N ^{+} _{i} by the identification (z ^{-} ,y) \equiv (z ^{+} ,f _{i(y)),} i = 1 , 2 . Since f₁ and f₂ are orientation-preserving diffeomorphisms of S¹, they are isotopic to the identity and the manifold obtained by this regluing operation is homeomorphic to M. The leaves of l{F ^{\prime} }, however, reassemble to produce a new foliation l{F} (f₁,f₂) of M. If a leaf L of l{F} (f₁,f₂) contains a piece Σ' ×, thenL = c up _{g} \Sigma ^{\prime} x \{ g (y _{0)} \}, where G ⊂ Diff₊(S¹) is the subgroup generated by . These copies of Σ' are attached to one another by identifications

(z⁻,g(y₀)) ≡ (z⁺,f₁(g(y₀))) for each z ∈ C₁,

(z⁻,g(y₀)) ≡ (z⁺,f₂(g(y₀))) for each z ∈ C₂,where g ranges over G. The leaf is completely determined by the G-orbit of y₀ ∈ S¹ and can he simple or immensely complicated. For instance, a leaf will be compact precisely if the corresponding G-orbit is finite. As an extreme example, if G is trivial (f₁ = f₂ = id_S¹), then

l{F} (f₁,f₂) = l{F} . If an orbit is dense in S¹, the corresponding leaf is dense in M. As an example, if f₁ and f₂ are rotations through rationally independent multiples of 2π, every leaf will be dense. In other examples, some leaf L has closure \bar{L} that meets each factor × S¹ in a Cantor set. Similar constructions can be made on Σ × I, where I is a compact, nondegenerate interval. Here one takes f₁,f₂ ∈ Diff₊(I) and, since ∂I is fixed pointwise by all orientation-preserving diffeomorphisms, one gets a foliation having the two components of ∂M as leaves. When one forms M' in this case, one gets a foliated manifold with corners. In either case, this construction is called the suspension of a pair of diffeomorphisms and is a fertile source of interesting examples of codimension-one foliations.

Foliations and integrability

There is a close relationship, assuming everything is smooth, with vector fields: given a vector field on that is never zero, its integral curves will give a 1-dimensional foliation. (i.e. a codimension foliation).

This observation generalises to the Frobenius theorem, saying that the necessary and sufficient conditions for a distribution (i.e. an dimensional subbundle of the tangent bundle of a manifold) to be tangent to the leaves of a foliation, is that the set of vector fields tangent to the distribution are closed under Lie bracket. One can also phrase this differently, as a question of reduction of the structure group of the tangent bundle from to a reducible subgroup.

The conditions in the Frobenius theorem appear as integrability conditions; and the assertion is that if those are fulfilled the reduction can take place because local transition functions with the required block structure exist. For example, in the codimension 1 case, we can define the tangent bundle of the foliation as, for some (non-canonical) (i.e. a non-zero co-vector field). A given is integrable iff everywhere.

There is a global foliation theory, because topological constraints exist. For example, in the surface case, an everywhere non-zero vector field can exist on an orientable compact surface only for the torus. This is a consequence of the Poincaré–Hopf index theorem, which shows the Euler characteristic will have to be 0. There are many deep connections with contact topology, which is the "opposite" concept, requiring that the integrability condition is never satisfied.

Existence of foliations

gave a necessary and sufficient condition for a distribution on a connected non-compact manifold to be homotopic to an integrable distribution. showed that any compact manifold with a distribution has a foliation of the same dimension.

See also

• • closed under taking pullbacks.
  • .

External links

• Foliations at the Manifold Atlas