InvariantEquation - Maple Help
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InvariantEquation

  

computes the differential equation invariant under a given symmetry group

 

Calling Sequence

Parameters

Description

Examples

Compatibility

Calling Sequence

InvariantEquation(S, DepVars, 'options'='value')

Parameters

S

-

the infinitesimals of a symmetry generator or the corresponding infinitesimal generator operator, or a list of n of them representing an n-dimensional group of symmetries

DepVars

-

a function or a list of them indicating the dependent variables of the problem

arbitraryfunctionname

-

(optional) the name used to represent the arbitrary function that appears in the returned invariant equation

explicit

-

(optional) to return the invariant equation(s) with the highest derivative(s) isolated on the left-hand-side(s)

order

-

(optional) the order of the differential equation required. The default value is 1.

jetnotation = ...

-

(optional) can be true (same as jetvariables), false (default), jetvariables, jetvariableswithbrackets, jetnumbers or jetODE; to respectively return or not using the different jet notations available

simplifier = ...

-

(optional) indicates the simplifier to use instead of the default simplify/size simplifier

Description

• 

Given a list with the infinitesimals of a symmetry generator or the corresponding infinitesimal generator operator, or a list of n of these lists or operators, possibly representing an n-dimensional symmetry group, InvariantEquation returns an expression , that is simultaneously invariant under all of the  symmetry generators () related to the given  infinitesimals. The  form a complete set of  differential invariants, so they are functions of the independent and dependent variables of the problem (DepVars), as well as their partial derivatives up to order (if not indicated, its default value is 1), and satisfy . The equation  is thus the most general partial differential equation simultaneously invariant under all the symmetries in S.

• 

InvariantEquation works also with dynamical symmetries, that is, symmetries where the infinitesimals themselves depend on derivatives of the dependent variables of the problem up to (at most) order - 1, where order is the parameter used to indicate the differential order of the desired invariant equation.

• 

Note that in the multidimensional case, depending on the form of the infinitesimals given, the problem may have no solution. Also the  entering  are all independent of each other and their number is unique, but one can still rewrite the  in different ways, by combining invariants to construct equivalent but algebraically different invariants of the same order.

• 

The representation  for the invariant equation can be transformed into a system of equations by constructing the objects

  

so that when the , isolated on the left-hand-side, is linear in the highest derivatives, of differential order equal to the value of order used by InvariantEquation, the resulting system will be an explicit PDE system linear in the highest derivatives.

• 

You can request InvariantEquation to directly return a similar explicit PDE system of differential order equal to order by passing the optional argument explicit. In that case the system returned:

– 

will contain as many differential equations as the number - say  - of partial derivatives of order equal to order;

– 

all the partial derivatives of order = order will appear on the left-hand-sides isolated;

– 

the right-hand-sides will involve  arbitrary functions of the form  with ;

– 

the right-hand-sides will depend on all the partial derivatives of at most one order less than order, including the unknowns themselves and the independent variables of the problem.

• 

This invariant PDE system is returned as an ordered list of equations, where the ordering is dictated by the partial derivatives and is the same implemented in the SortDerivatives command of the PDEtools Library programming routines.

• 

The returned invariant equation is automatically simplified in size. To avoid that, pass the optional argument simplifier = none, or to use a different simplifier use simplifier = ....

• 

By default, the equation returned by InvariantEquation is in function notation. You can change this by specifying the optional argument jetnotation = ... where the value can be any of the jet notations available.

Examples

Consider a PDE problem with two independent variables and one dependent variable, , and consider the list of infinitesimals of a symmetry group.

(1)

In the input above you can also enter the symmetry  without labels for the infinitesimals, as in , or use the corresponding infinitesimal generator

(2)

The equation invariant under the symmetry transformation underlying the infinitesimal generator  is (you can equally use G instead of S)

(3)

This is the explicit form of the PDE system as explained in the Description, in terms of two arbitrary functions  and

(4)

The invariance of  can be verified in various ways, the simplest of which is perhaps to use SymmetryTest, which verifies that  is a symmetry of .

(5)

A less abstract verification would be to explicitly construct the symmetry transformation related to , for example, in terms of new variables .

(6)

(7)

Now, change the variables in  using , and recall that the Lie group parameter  is real:

(8)

(9)

The equation above is identical to , that is the meaning of invariant in the context of symmetries. Alternatively, the following is the prolongation to order 1 of the infinitesimal generator  (5.2) (that is, ready to act on functions depending on  and partial derivatives of  of order 1 at most).

(10)

If you apply this differential operator  to , you obtain zero because of the invariance of the  under the transformation  related to . To apply , an operator in jet notation, you need to rewrite  in the same notation

(11)

(12)

Underlying this zero, there is the way  is constructed: it is an arbitrary function of the Invariants associated to , that is, the solutions of the differential operator (you can equally pass  instead of )

(13)

InvariantEquation also handles dynamical symmetries, that is, symmetries that depend on the partial derivatives of the dependent variables of the problem. In these cases it is however of no use to directly compute the invariants, as in the example above, because they will all be of higher order. The approach used is then different. Consider for instance these infinitesimals depending on

(14)

As mentioned, the differential invariants, for instance, of order 1, automatically come depending on higher derivatives

(15)

In fact, for order = n, and besides the invariant of order 0, all the other ones will always be of order . To see the actual invariants of order 1 you would need to eliminate the higher derivatives using the related invariant equation, thus defeating the use of these invariants with the purposes of constructing invariant equations of order . The approach used by InvariantEquation results instead in

(16)

Note the  depend on first and second derivatives of , as it should be. In explicit form (use declare to have a more compact and readable output with derivatives displayed in jet notation)

(17)

(18)

where the PDE system returned contains as many differential equations as partial derivatives of order 2, that is, 3 equations, and the same number of arbitrary functions .

Compatibility

• 

The explicit option was introduced in Maple 15.

• 

For more information on Maple 15 changes, see Updates in Maple 15.

See Also

dchange

InfinitesimalGenerator

Invariants

PDEtools

SymmetryTest

SymmetryTransformation

ToJet

 


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