Definition
Euler subsolution with prescribed energy
A linear momentum solution whose velocity and trace-free flux lie below a prescribed energy matrix.
Let be locally integrable on a space-time cylinder in dimension . An Euler subsolution with energy bound is a triple , with , symmetric and trace free, and a distribution, such that
The equations use distributional derivatives; the inequality says that is positive semidefinite almost everywhere. A smooth strict subsolution has smooth , continuous , and a positive definite gap everywhere.
The products are outer products, the coefficients have the indicated local integrability, and the pressure is a distribution. This formulation relaxes Euler while keeping its divergence constraints.
Recovering Euler
Taking the trace gives . If equality holds almost everywhere, the positive semidefinite gap has zero trace and is zero. Then , and satisfies Euler with pressure .