# Benjamin-Bona-Mahony equation

BBM equation, regularized long wave equation

The model equation

 (a1)

where and the subscripts denote partial derivatives with respect to time and the position coordinate . The Benjamin–Bona–Mahony equation serves as an approximate model in studying the dynamics of small-amplitude surface water waves propagating unidirectionally, while suffering non-linear and dispersive effects. (a1) was introduced in [a5] as an alternative of the famous Korteweg–de Vries equation

 (a2)

Unlike the Korteweg–de Vries equation, the Benjamin–Bona–Mahony equation is not integrable by the inverse scattering method [a10], [a12]. As indicated by several numerical experiments, (a1) has no multi-soliton solutions. It has been proved by A.C. Bryan and A.E.G. Stuart [a8] that (a1) has no analytic two-soliton solution. The equation has three independent invariants (conservation laws):

;

; and

. These quantities are time-independent during the time evolution of the solution . The correctness of the initial value problem (the Cauchy problem) for (a1) in Sobolev spaces , (cf. also Sobolev space), was investigated in [a5].

Equation (a1) has a solitary wave solution , where (cf. also Soliton), provided that the wave velocity satisfies . The non-linear stability of the wave with respect to the pseudo-metric was established in [a3] and [a7] by a clever modification of Lyapunov's direct method in combination with a spectral decomposition technique. Here, is the norm in the Sobolev space . This means that the form of the solitary wave is stable under small perturbations in the form of the initial wave.

## Generalizations.

The generalized Benjamin–Bona–Mahony equation is an equation of the form

 (a3)

where is a differentiable function. (a3) allows two types of solitary waves: kink-shaped and bell-shaped ones. Depending on the concrete form of the non-linearity, these solitary waves can be stable or unstable with respect to the metric . For more concrete results concerning (a3), see [a11], Chap. 4. The generalized Benjamin–Bona–Mahony equation in higher dimensions reads

 (a4)

where is the Laplace operator in and . The uniqueness and global existence of a solution in Sobolev spaces to the initial boundary value problem for (a4) in , with Dirichlet (or more general) boundary conditions, was proved in [a2], [a9]. Here, is a bounded domain with smooth boundary. The Cauchy problem for (a4) is studied in [a1].

Non-local generalizations of the Benjamin–Bona–Mahony equation can be obtained after one writes (a1) in the form

Here, is a pseudo-differential operator (in fact, a Fourier multiplier operator), acting as , where denotes the Fourier transform in the space variable. For the original Benjamin–Bona–Mahony equation one has . In general, one takes for a positive even function such that its negative power is monotone decreasing on and belongs to . See [a4], [a5] and the references therein for more details.

The variable-coefficient Benjamin–Bona–Mahony equation

describes the propagation of long weakly non-linear water waves in a channel of variable depth. This equation was studied in [a6].

#### References

 [a1] J. Avrin, "The generalized Benjamin–Bona–Mahony equation in with singular initial data" Nonlin. Anal. Th. Meth. Appl. , 11 (1987) pp. 139–147 [a2] J. Avrin, J.A. Goldstein, "Global existence for the Benjamin–Bona–Mahony equation in arbitrary dimensions" Nonlin. Anal. Th. Meth. Appl. , 9 (1985) pp. 861–865 [a3] T.B. Benjamin, "The stability of solitary waves" Proc. Royal Soc. London A , 328 (1972) pp. 153–183 [a4] T.B. Benjamin, "Lectures on nonlinear wave motion" A.C. Newell (ed.) , Nonlinear Wave Motion , Lectures in Applied Math. , 15 , Amer. Math. Soc. (1974) pp. 3–47 [a5] T.B. Benjamin, J.L. Bona, J.J. Mahony, "Model equations for long waves in nonlinear dispersive systems" Philos. Trans. Royal Soc. London A , 272 (1972) pp. 47–78 [a6] V. Bisognin, G. Perla Menzala, "Asymptotic behaviour of nonlinear dispersive models with variable coefficients" Ann. Mat. Pura Appl. , 168 (1995) pp. 219–235 [a7] J.L. Bona, "On the stability theory of solitary waves" Proc. Royal Soc. London A , 344 (1975) pp. 363–374 [a8] A.C. Bryan, A.E.G. Stuart, "Solitons and the regularized long wave equation: a nonexistence theorem" Chaos, Solitons, Fractals , 7 (1996) pp. 1881–1886 [a9] B. Calvert, "The equation " Math. Proc. Cambridge Philos. Soc. , 79 (1976) pp. 545–561 [a10] C.S. Gardner, J.M. Greene, M.D. Kruskal, R.M. Miura, "Method for solving the Korteweg–de Vries equation" Phys. Rev. Lett. , 19 (1967) pp. 1095–1097 [a11] I.D. Iliev, E. Khristov, K.P. Kirchev, "Spectral methods in soliton equations" , Pitman Monographs and Surveys Pure Appl. Math. , 73 , Longman (1994) [a12] P.D. Lax, "Integrals of nonlinear equations of evolution and solitary waves" Commun. Pure Appl. Math. , 21 (1968) pp. 467–490
How to Cite This Entry:
Benjamin-Bona-Mahony equation. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Benjamin-Bona-Mahony_equation&oldid=22089
This article was adapted from an original article by Iliya D. Iliev (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article