# Scattering for NLW/NLKG

The Strauss exponent

${\displaystyle p_{0}(d)=[d+2+{\sqrt {d^{2}+12d+4}}]/2d}$

plays a key role in the GWP and scattering theory. We have ${\displaystyle p_{0}(1)=[3+{\sqrt {17}}]/2}$; ${\displaystyle p_{0}(2)=1+sqrt(2);p_{0}(3)=2}$; note that ${\displaystyle p_{0}(d-1)}$ is always between the ${\displaystyle L^{2}}$ and ${\displaystyle H^{1/2}}$ critical powers, and ${\displaystyle p_{0}(d)}$ is always between the ${\displaystyle H^{1/2}}$ and ${\displaystyle H^{1}}$ critical powers.

Another key power is

${\displaystyle p_{*}(d)=[d+2+sqrt(d^{2}+8d)]/2(d-1)}$

which lies between the ${\displaystyle L^{2}}$ critical power and ${\displaystyle p_{0}(d-1)}$.

Caveats: the ${\displaystyle d=1,2}$ cases may be somewhat different from what is stated here (partly because some of the powers here are not well-defined). Also, in many of the NLW results one needs some additional decay at spatial infinity (e.g. finiteness of the conformal energy), except in the special ${\displaystyle H^{1}}$-critical case. This is because (unlike NLS and NLKG) there is no a priori bound on the ${\displaystyle L^{2}}$ norm (even with conservation of energy).

Scattering for small ${\displaystyle H^{1}}$ data for arbitrary NLW:

• Known for ${\displaystyle p_{*}(d) Sr1981.
• For ${\displaystyle p one has blow-up Si1984.
• When ${\displaystyle d=3}$ this is extended to ${\displaystyle 5/2, but scattering fails for ${\displaystyle p<5/2}$ Hi-p3
• When ${\displaystyle d=4}$ this is extended to ${\displaystyle p_{0}(d-1)=2, but scattering fails for ${\displaystyle p<2}$ Hi-p3
• An alternate argument based on conformal compactification but giving slightly different results are in BcKkZz1999

Scattering for large ${\displaystyle H^{1}}$ data for defocusing NLW:

• Known for ${\displaystyle p_{H^{1/2}} BaSa1998, BaGd1997 (GWP was established earlier in GiVl1987).
• Known for ${\displaystyle p=p_{H^{1/2}}}$, ${\displaystyle d=3}$ BaeSgZz1990
• When ${\displaystyle d=3}$ this is extended to ${\displaystyle p_{*}(3) Hi-p3
• When ${\displaystyle d=4}$ this is extended to ${\displaystyle p_{*}(4) Hi-p3
• For ${\displaystyle d>4}$ one expects scattering when ${\displaystyle p_{0}(d-1), but this is not known.

Scattering for small smooth compactly supported data for arbitrary NLW:

• GWP and scattering when ${\displaystyle p>p_{0}(d-1)}$ GeLbSo1997
• For ${\displaystyle d=3}$ this is in Jo1979
• Blow-up for arbitrary nonzero data when ${\displaystyle p Si1984 (see also Rm1987, JiZz2003
• For ${\displaystyle d=4}$ this is in Gs1981b
• For ${\displaystyle d=3}$ this is in Jo1979
• At the critical power ${\displaystyle p=p_{0}(d-1)}$ there is blowup for non-negative non-trivial data YoZgq-p2
• For ${\displaystyle d=2,3}$ and arbitrary nonzero data this is in Scf1985
• For large data and arbitrary ${\displaystyle d}$ this is in Lev1990

Scattering for small ${\displaystyle H^{1}}$ data for arbitrary NLKG:

• Decay estimates are known when ${\displaystyle p_{0}(d)MsSrWa1980, Br1984, Sr1981, Pe1985.
• Known when ${\displaystyle p_{L^{2}}\leq p\leq p_{H^{1}}}$ Na1999c, Na1999d, Na2001. Indeed, one has existence of wave operators and asymptotic completeness in these cases.

Scattering for large ${\displaystyle H^{1}}$ data for defocussing NLKG:

• In this case one has an a priori ${\displaystyle L^{2}}$ bound and one does not need decay at spatial infinity.
• Scattering is known for ${\displaystyle p_{L^{2}} Na1999c, Na1999d, Na2001
• For ${\displaystyle d>2}$ and ${\displaystyle p}$ not ${\displaystyle H^{1}}$-critical this is in Br1985 GiVl1985b
• The ${\displaystyle L^{2}}$-critical case ${\displaystyle p=p_{L^{2}}}$ is an interesting open problem.

Scattering for small smooth compactly supported data for arbitrary NLKG:

• GWP and scattering for ${\displaystyle p>1+2/d}$ when ${\displaystyle d=1,2,3}$ LbSo1996
• When ${\displaystyle d=1,2}$ this can be obtained by energy estimates and decay estimates.
• In principle this extends to higher dimensions but there is a difficulty with lack of smoothness in the nonlinearity.
• Blowup in the non-Hamiltonian case when ${\displaystyle p<1+2/d}$ KeTa1999. The endpoint ${\displaystyle p=1+2/d}$ remains open but one probably also has blow-up here.
• Failure of scattering for ${\displaystyle p\leq 1+2/d}$ was shown in Gs1973.

An interesting (and apparently under-explored) problem is what happens to these global existence and scattering results when there is an obstacle. For NLW-5 on ${\displaystyle R^{3}}$ one has global regularity for convex obstacles SmhSo1995, and for smooth non-linearities there is the general quasilinear theory. If one adds a suitable damping term near the obstacle then one can recover some global existence results Nk2001.

On the Schwarzschild manifold some scattering and decay results for NLW and NLWKG can be found in BchNic1993, Nic1996, BluSf2003