Flow (mathematics)

In mathematics, a flow formalizes the idea of the motion of particles in a fluid. Flows are ubiquitous in science, including engineering and physics. The notion of flow is basic to the study of ordinary differential equations. Informally, a flow may be viewed as a continuous motion of points over time. More formally, a flow is a group action of the real numbers on a set.
The idea of a vector flow, that is, the flow determined by a vector field, occurs in the areas of differential topology, Riemannian geometry and Lie groups. Specific examples of vector flows include the geodesic flow, the Hamiltonian flow, the Ricci flow, the mean curvature flow, and Anosov flows. Flows may also be defined for systems of random variables and stochastic processes, and occur in the study of ergodic dynamical systems. The most celebrated of these is perhaps the Bernoulli flow.
Formal definition.
A flow on a set is a group action of the additive group of real numbers on . More explicitly, a flow is a mapping
such that, for all ∈ and all real numbers and ,
It is customary to write instead of , so that the equations above can be expressed as (the identity function) and (group law). Then, for all , the mapping is a bijection with inverse . This follows from the above definition, and the real parameter may be taken as a generalized functional power, as in function iteration.
Flows are usually required to be compatible with structures furnished on the set . In particular, if is equipped with a topology, then is usually required to be continuous. If is equipped with a differentiable structure, then is usually required to be differentiable. In these cases the flow forms a one parameter subgroup of homeomorphisms and diffeomorphisms, respectively.
In certain situations one might also consider local flows, which are defined only in some subset
called the flow domain of . This is often the case with the flows of vector fields.
Alternative notations.
It is very common in many fields, including engineering, physics and the study of differential equations, to use a notation that makes the flow implicit. Thus, is written for , and one might say that the "variable depends on the time and the initial condition ". Examples are given below.
In the case of a flow of a vector field on a smooth manifold , the flow is often denoted in such a way that its generator is made explicit. For example,
Orbits.
Given in , the set formula_6 is called the orbit of under . Informally, it may be regarded as the trajectory of a particle that was initially positioned at . If the flow is generated by a vector field, then its orbits are the images of its integral curves.
Examples.
Algebraic equation.
Let be a time-dependent trajectory which is a bijective function, i.e, non-periodic function. Then a flow can be defined by
Autonomous systems of ordinary differential equations.
Let be a (time-independent) vector field
and the solution of the initial value problem
Then is the flow of the vector field "F". It is a well-defined local flow provided that the vector field 
 is Lipschitz-continuous. Then is also Lipschitz-continuous wherever defined. In general it may be hard to show that the flow is globally defined, but one simple criterion is that the vector field F is compactly supported.
Time-dependent ordinary differential equations.
In the case of time-dependent vector fields , one denotes , where is the solution of
Then is the time-dependent flow of . It is not a "flow" by the definition above, but it can easily be seen as one by rearranging its arguments. Namely, the mapping
indeed satisfies the group law for the last variable:
One can see time-dependent flows of vector fields as special cases of time-independent ones by the following trick. Define
Then is the solution of the "time-independent" initial value problem
if and only if is the solution of the original time-dependent initial value problem. Furthermore, then the mapping is exactly the flow of the "time-independent" vector field .
Flows of vector fields on manifolds.
The flows of time-independent and time-dependent vector fields are defined on smooth manifolds exactly as they are defined on the Euclidean space and their local behavior is the same. However, the global topological structure of a smooth manifold is strongly manifest in what kind of global vector fields it can support, and flows of vector fields on smooth manifolds are indeed an important tool in differential topology. The bulk of studies in dynamical systems are conducted on smooth manifolds, which are thought of as "parameter spaces" in applications.
Solutions of heat equation.
Let be a subdomain (bounded or not) of (with an integer). Denote by its boundary (assumed smooth). 
Consider the following heat equation on × (0,), for ,
with the following initial boundary condition in .
The equation = 0 on corresponds to the Homogeneous Dirichlet boundary condition. The mathematical setting for this problem can be the semigroup approach. To use this tool, we introduce the unbounded operator defined on formula_21 by its domain
(see the classical Sobolev spaces with formula_23 
and 
is the closure of the infinitely differentiable functions with compact support in for the formula_25norm). 
For any formula_26, we have
With this operator, the heat equation becomes formula_28 and . Thus, the flow corresponding to this equation is (see notations above)
where is the (analytic) semigroup generated by .
Solutions of wave equation.
Again, let be a subdomain (bounded or not) of (with an integer). We denote by its boundary (assumed smooth). 
Consider the following wave equation on formula_30 (for ),
with the following initial condition in formula_32 and formula_33.
Using the same semigroup approach as in the case of the Heat Equation above. We write the wave equation as a first order in time partial differential equation by introducing the following unbounded operator,
with domain formula_35 on formula_36 (the operator formula_37 is defined in the previous example). 
We introduce the column vectors 
(where formula_39 and formula_40) and 
With these notions, the Wave Equation becomes formula_42 and formula_43. 
Thus, the flow corresponding to this equation is 
formula_44
where formula_45 is the (unitary) semigroup generated by formula_46.
Bernoulli flow.
Ergodic dynamical systems, that is, systems exhibiting randomness, exhibit flows as well. The most celebrated of these is perhaps the Bernoulli flow. The Ornstein isomorphism theorem states that, for any given entropy , there exists a flow , called the Bernoulli flow, such that the flow at time , "i.e." , is a Bernoulli shift.
Furthermore, this flow is unique, up to a constant rescaling of time. That is, if , is another flow with the same entropy, then , for some constant . The notion of uniqueness and isomorphism here is that of the isomorphism of dynamical systems. Many dynamical systems, including Sinai's billiards and Anosov flows are isomorphic to Bernoulli shifts.