In mathematical physics, the Degasperis–Procesi equation

${\displaystyle \displaystyle u_{t}-u_{xxt}+2\kappa u_{x}+4uu_{x}=3u_{x}u_{xx}+uu_{xxx}}$

is one of only two exactly solvable equations in the following family of third-order, non-linear, dispersive PDEs:

${\displaystyle \displaystyle u_{t}-u_{xxt}+2\kappa u_{x}+(b+1)uu_{x}=bu_{x}u_{xx}+uu_{xxx},}$

where ${\displaystyle \kappa }$ and b are real parameters (b=3 for the Degasperis–Procesi equation). It was discovered by Degasperis and Procesi in a search for integrable equations similar in form to the Camassa–Holm equation, which is the other integrable equation in this family (corresponding to b=2); that those two equations are the only integrable cases has been verified using a variety of different integrability tests.^[1] Although discovered solely because of its mathematical properties, the Degasperis–Procesi equation (with ${\displaystyle \kappa >0}$) has later been found to play a similar role in water wave theory as the Camassa–Holm equation.^[2]

Among the solutions of the Degasperis–Procesi equation (in the special case ${\displaystyle \kappa =0}$) are the so-called multipeakon solutions, which are functions of the form

${\displaystyle \displaystyle u(x,t)=\sum _{i=1}^{n}m_{i}(t)e^{-|x-x_{i}(t)|}}$

where the functions ${\displaystyle m_{i}}$ and ${\displaystyle x_{i}}$ satisfy^[3]

${\displaystyle {\dot {x}}_{i}=\sum _{j=1}^{n}m_{j}e^{-|x_{i}-x_{j}|},\qquad {\dot {m}}_{i}=2m_{i}\sum _{j=1}^{n}m_{j}\,\operatorname {sgn} {(x_{i}-x_{j})}e^{-|x_{i}-x_{j}|}.}$

These ODEs can be solved explicitly in terms of elementary functions, using inverse spectral methods.^[4]

When ${\displaystyle \kappa >0}$ the soliton solutions of the Degasperis–Procesi equation are smooth; they converge to peakons in the limit as ${\displaystyle \kappa }$ tends to zero.^[5]

The Degasperis–Procesi equation (with ${\displaystyle \kappa =0}$) is formally equivalent to the (nonlocal) hyperbolic conservation law

${\displaystyle \partial _{t}u+\partial _{x}\left[{\frac {u^{2}}{2}}+{\frac {G}{2}}*{\frac {3u^{2}}{2}}\right]=0,}$

where ${\displaystyle G(x)=\exp(-|x|)}$, and where the star denotes convolution with respect to x. In this formulation, it admits weak solutions with a very low degree of regularity, even discontinuous ones (shock waves).^[6] In contrast, the corresponding formulation of the Camassa–Holm equation contains a convolution involving both ${\displaystyle u^{2}}$ and ${\displaystyle u_{x}^{2}}$, which only makes sense if u lies in the Sobolev space ${\displaystyle H^{1}=W^{1,2}}$ with respect to x. By the Sobolev embedding theorem, this means in particular that the weak solutions of the Camassa–Holm equation must be continuous with respect to x.