# QNLW

In a local co-ordinate chart, **quasilinear wave equations** (QNLW) take the form

One could also consider equations where the metric depends on derivatives of , but one can reduce to this case (giving up a derivative) by differentiating the equation. One can also reduce to the case , by a suitable change of variables. is usually quadratic in the derivatives , as this formulation is then robust under many types of changes of variables.

## Specific equations

Quasilinear NLWs appear frequently in general relativity. Examples include

- The Einstein equations
- Equations of relativistic elasticity
- Equations of relativistic hydrodynamics
- The minimal surface equation

The most interesting dimension is of course the physical dimension d=3.

## Wellposedness theory

Classically one has LWP for when (HuKaMar1977), but the semilinear theory suggests that we should be able to improve this to with a null condition, and to without one (these results would be sharp even in the semilinear case). In principle Strichartz estimates should be able to push down to , but only partial results of this type are known. Specifically:

- When one has LWP in the expected range without a null condition (SmhTt-p).
- When one has LWP for SmhTt-p (using parametrices and the equation for the metric); in the specific case of the Einstein equations see KlRo-p3, KlRo-p4, KlRo-p5 (using vector fields and the equation for the metric)

A special type of QNLW is the cubic equations, where the metric g itself obeys an elliptic equation of the form Delta , and the non-linearity is of the form . For such equations, we have LWP for when BaCh 2003, BaCh2002. This equation has some similarity with the differentiated wave maps equation in the Coulomb gauge.

For small smooth compactly supported data of size and smooth non-linearities, the GWP theory for QNLW is as follows.

- If the non-linearity is a null form, then one has GWP for ; in fact one can take the data in a weighted Sobolev space Cd1986.
- Without the null structure, one has almost GWP in d=3 Kl1985b, and this is sharp Jo1981, Si1983
- In the semi-linear case and when the nonlinearity is quadratic in the derivatives, this is also true outside of a compact non-trapping obstacle KeSmhSo-p2. This has been generalized to the quasi-linear case in KeSmhSo-p3 (and non-linear Dirichlet wave equations are also treated there, as are multiple speeds).

- With a null structure and outside a star-shaped obstacle with Dirichlet conditions and , one has GWP for small data in which are compatible with the boundary [KeSmhSo-p]. Earlier work in this direction is in Dt1990.
- For radial data and obstacle this was obtained in Go1995; see also Ha1995, Ha2000.
- In the semilinear case, the non-trapping condition was removed in MetSo-p, even in the multiple speed case, provided one has an exponential decay result near the obstacle (this is true, for instance, if the obstacle is a union of a finite number of sufficiently separated strictly convex bodies).

- For or for cubic nonlinearities one has GWP regardless of the null structure KlPo1983, Sa1982, Kl1985b.
- In three dimensions with a null structure, for systems with multiple wave speeds, one has GWP So2001
- In the exterior of an nontrapping obstacle with Dirichlet conditions, with multiple speeds, one has GWP for sufficiently smooth and decaying data obeying the usual compatibility conditions at the boundary MetSo-p2, if the quasilinear terms obey some symmetry conditions and the semilinear terms are quadratic in

- Without the null structure, one has almost GWP in d=3 Kl1985b, and this is sharp Jo1981, Si1983