Riemann invariants are transformations made on a a system of quasi-linear first order partial differential equations(pdes) to make them more easily solvable. Riemann invariants are constant along the the characteristic curves of the partial differential equaitons where they obtain the name invariant.
They were first obtained by Bernhard Riemann in his work on plane waves in gas dynamics(1858).
where and are the elements of the matrices A and a and where and are elements of vectors.
It will be asked if it is possible to rewrite this equation to
To do this curves wil be introduced in the (x,t) plane defined by the vector field . The term in the brackets will be rewritten interms of a total derivative where x,t are parameterised as
comparing the last two equations we find
which can be now written in characteristic form
where we must have the conditions
,
where ca be eliminated to give the necessary condition
so for a nontrival solution is the determinate
For Riemann invariants we are concerned with the case when the matrix A is an identity matrix to form
notice this is homogeneous due to the vector b being zero. In cahracteristic form the system is
with
Where l the left eigenvector of the matrix A and the the characteristic speeds are the eigenvalues of the matrix A which satisfy
To simplify these characteristic equations we can make the transformations such that
which form
An integrating factor can be multiplied in to help integrate this. So the system now has the characteristic form
^ ab "Nonlinear Periodic Waves and their Modulations",A.M. Kamchatnov (2000),World Scientific . Cite error: The named reference "multiple" was defined multiple times with different content (see the help page).
^ Poisson brackets and the one-dimaentional Hamiltonian systems of the hydrodynamic type,S.P. Tsarev,1985