Consider an n × n hyperbolic system of conservation laws of the form
ut + f(u)x = 0, (t,x) ∈ ℝ+ × ℝ, u ∈ ℝn.
Here u = (u1,…,un) is the vector of conserved quantities, while the components of f = (f1,…,fn) are the luxes. The system is said strictly hyperbolic if at each point u the Jacobian matrix Df(u) has n real, distinct eigenvalues
λ1(u)< ··· < λn(u).
A fundamental ingredient to prove existence and stability in BV is the introduction of a functional, the Glimm–Liu interaction functional, which controls the interactions among non linear waves.
Aim of this note is to present a simple interpretation of (the scalar part of) the interaction functional, and show how it can be extended to the following equation:
a parabolic equation of the form
ut + f(u)x = uxx;
scalar semidiscrete schemes, for example the upwind scheme
ut(t,x) + (f(u(t,x)) − f(u(t,x −1)) = 0,
or the backward scheme
(u(t,x) − u(t −1,x)) + f(u(t,x)) = 0;
2 × 2 relaxation approximation, in particular
ut + vx = 0,
vt + ux = f(u) − v.
All these approximations are interesting from the physical and numerical point of view. Finding an interaction is one of the key steps toward the proof of BV bounds.