# Difference between revisions of "Benjamin-Ono equation"

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Benjamin-Ono equation

The Benjamin-Ono equation (BO) Bj1967, On1975, which models one-dimensional internal waves in deep water, is given by

${\displaystyle u_{t}+Hu_{xx}+uu_{x}=0}$

where H is the Hilbert transform. This equation is completely integrable (see e.g. AbFs1983, CoiWic1990).

Scaling is s = -1/2, and the following results are known:

## Generalized Benjamin-Ono equation

The generalized Benjamin-Ono equation is the scalar equation

${\displaystyle \partial _{t}u+D_{x}^{1+a}\partial _{x}u+u\partial _{x}u=0.}$

where ${\displaystyle D_{x}:={\sqrt {-\Delta }}}$ is the positive differentiation operator. When a=1 this is KdV; when a=0 this is Benjamin-Ono. Both of these two extreme cases are completely integrable, though the intermediate cases 0 < a < 1 are not.

When 0 < a < 1, scaling is s = -1/2 - a, and the following results are known:

• LWP in H^s is known for s > 9/8 - 3a/8 KnKoe2003
• GWP is known when s >= (a+1)/2 when a > 4/5, from the conservation of the Hamiltonian KnPoVe1994b
• The LWP results are obtained by energy methods; it is known that pure iteration methods cannot work MlSauTz2001
• However, this can be salvaged by combining the H^s norm || f ||_{H^s} with a weighted Sobolev space, namely || xf ||_{H^{s - 2s_*}}, where s_* = (a+1)/2 is the energy regularity. CoKnSt2003

## Benjamin-Ono with power nonlinearity

This is the equation

${\displaystyle u_{t}+Hu_{xx}+(u^{k})_{x}=0.}$

Thus the original Benjamin-Ono equation corresponds to the case k=2. The scaling exponent is 1/2 - 1/(k-1).

• For k=3, one has GWP for large data in H^1 KnKoe2003 and LWP for small data in H^s, s > ½ MlRi2004
• For small data in H^s, s>1, LWP was obtained in KnPoVe1994b
• With the addition of a small viscosity term, GWP can also be obtained in H^1 by complete integrability methods in FsLu2000, with asymptotics under the additional assumption that the initial data is in L^1.
• For s < ½, the solution map is not C^3 MlRi2004
• For k=4, LWP for small data in H^s, s > 5/6 was obtained in KnPoVe1994b.
• For k>4, LWP for small data in H^s, s >=3/4 was obtained in KnPoVe1994b.
• For any k >= 3 and s < 1/2 - 1/k the solution map is not uniformly continuous BiLi2001

## Other generalizations

The KdV-Benjamin Ono equation is formed by combining the linear parts of the KdV and Benjamin-Ono equations together. It is globally well-posed in L^2 Li1999, and locally well-posed in H^{-3/4+} KozOgTns2001 (see also HuoGuo2005 where H^{-1/8+} is obtained).

Similarly one can generalize the non-linearity to be k-linear, generating for instance the modified KdV-BO equation, which is locally well-posed in H^{1/4+} HuoGuo2005. For general gKdV-gBO equations one has local well-posedness in H^3 and above GuoTan1992. One can also add damping terms Hu_x to the equation; this arises as a model for ion-acoustic waves of finite amplitude with linear Landau damping OttSud1970.