computes the conserved currents of a system of ordinary or partial differential equations (PDE)
tests whether a given list of expressions is a list of conserved currents of the given PDE system
ConservedCurrents(PDESYS, DepVars, _J = ..., displayfunctionality = ..., jetnotation = ..., simplifier = ..., split = ...)
ConservedCurrentTest(J, PDESYS, DepVars)
a system involving partial and/or ordinary (possibly not) differential equations
a conserved current returned by ConservedCurrents
(optional) a specification of the unknown(s) in PDESYS
_J = ...
(optional) indicates the functional form for the conserved currents
displayfunctionality = ...
(optional) true (default) or false, to display the functionality on the left-hand-side of the Jk (conserved currents) functions
jetnotation = ...
(optional) can be false (default), jetvariables, jetvariableswithbrackets, jetnumbers or jetODE; to respectively return or not using the different jet notations available
(optional) indicates the maximum differential order of the derivatives entering the dependence of the conserved currents
simplifier = ...
(optional) indicates the simplifier to be used instead of the default simplify/size
split = ...
(optional) true (default) or false, to split the DE system to be solved in order to compute the Jk functions
typeofconservedcurrent = ...
optional - can be polynomial or functionfield.
The command ConservedCurrents computes conserved currents for a given differential equation system PDESYS. Given a system Δ=0 (PDESYS) consisting of N equations pden, n=1..N, involving M independent variables x1,x2,...=X, and where the dependent variables are u1,u2,...=U, with dU denoting the set of partial derivatives of U, conserved currents are expressions Jα,m⁡X,U,dU each of which satisfy: d/d⁢x1⁢Jα,1⁡X,U,dU+d/d⁢x2⁢Jα,2⁡X,U,dU+...=Divergence⁡Jα,m = 0, where m=1..M and d/d⁢xm represents the total derivative with respect to xm. The conserved currents coincide with the traditional first integrals when there is only one independent variable, so that Δ is a system of ODEs.
The command ConservedCurrentTest verifies whether a given expression J is a conserved current of the system PDESYS. Note: Herein, Divergence⁡Jn=0 is assumed to mean that the left hand side is taken modulo the given PDE system; i.e. Divergence⁡Jn may result in a function of Δ that becomes zero whenever Δ=0.
The conserved currents Jα⁡X,U,dU are computed by first constructing the PDE system Divergence⁡Jα⁡X,U,dU=0 modulo PDESYS that they satisfy (see DeterminingPDE), then solving this system using pdsolve.
By default, the conserved currents are searched as functions depending on the derivatives of the unknowns of the system (they are automatically detected or can be specified as DepVars) up to the order d-1, where d is the highest order of derivatives entering PDESYS. This default can be changed by optionally passing the argument order = m, where m is a nonnegative integer.
By default, the conserved currents are searched as functions with no pre-specified form, just with the dependency explained in the previous paragraph. This default can be changed with the option typeofconservedcurrent = ... where the right-hand-side can be polynomial or functionfield, respectively indicating a conserved current of polynomial type, or of a functionfield type with the meaning explained in FunctionField.
By default, the functionality of Jα,m⁡X,U,dU, entering the left-hand-sides of each element in the returned lists, is displayed, the output is presented in functional notation instead of jet notation and is simplified with respect to its size. The PDE system solved to compute the Jα,m⁡X,U,dU is split, when that is possible, before being tackled. All these defaults can be changed by passing the optional arguments displayfunctionality = ..., jetnotation = ..., simplifier = ..., split = false.
It is also possible to directly specify the functionality expected for the Jα,m using _J = .... See the examples for a demonstration of the use of this parameter.
To avoid having to remember the optional keywords, if you type the keyword misspelled, or just a portion of it, a matching against the correct keywords is performed, and when there is only one match, the input is automatically corrected.
Consider the following PDE "system" consisting of a single PDE
U := diff_table(u(x,t)):
u⁡x,t⁢will now be displayed as⁢u
pde := U[t,t] + U[x,x] + U[x]*U;
Two conserved currents depending on arbitrary functions are
J[alpha] := ConservedCurrents(pde);
To verify these results use
map(ConservedCurrentTest, [J[alpha]], pde);
To compute the same result using different jet notations use the jetnotation option.
ConservedCurrents(pde, jetnotation = jetnumbers);
In various cases it is simpler, or of more use, to compute a conserved current of polynomial type, or with a mathematical function dependency on the field of functions of the input system. For these purposes use the option typeofconservedcurrent = ... where the right-hand-side can be polynomial or functionfield. For example, for pde1, a polynomial conserved current, presented without specializing the arbitrary constants (option split = false) is
J[polynomial] := ConservedCurrents(pde, type = polynomial, split = false);
* Partial match of 'type' against keyword 'typeofconservedcurrent'
Conserved currents are related to generalized integrating factors μα via Σ⁢μα,n⁢pden=Divergence⁡Jα = 0. These are the μα corresponding to the Jα computed above.
mu[alpha] := IntegratingFactors(pde);
Note that pde1 is already the divergence of a function, so that a constant (the number 1 in the result above) is an integrating factor. To verify for correctness these integrating factors use
map(IntegratingFactorTest, [mu[alpha]], pde);
Alternatively, applying Euler's operator to pde1 you see that pde1 is already a divergence.
An example where the integrating factor depends on an arbitrary function:
pde := U[x,t] + U[x,x,x] + U[x]*U = 0;
This is a related conserved current of order 1
ConservedCurrents(pde, order = 1);
This is a conserved current depending only on x and t; to compute it use the optional argument _j = ...(functional form desired)..
ConservedCurrents(pde, _J = f(x,t));
 Olver, P.J. Applications of Lie Groups to Differential Equations. Graduate Texts in Mathematics. Springer-Verlag, 1993.
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