1.
Introduction
Applied sciences rely heavily on closed-form mathematical theories. Much of such studies represents mathematical intuitions and skills of high order, challenging even for advanced researchers of today. A kind of multiple wave solutions, called soliton solutions, are examples of closed-form solutions to integrable models of nonlinear dispersive waves. The nonlinearity and the dispersion play together in generating such nonlinear dispersive wave solutions.
In soliton theory, there are two powerful techniques, the inverse scattering transform [1] and the Hirota bilinear method [2], to soliton solutions. The inverse scattering transform was developed initially for solving Cauchy problems of nonlinear model equations, genearted from Lax pairs of matrix spectral problems [3,4], It is a nonlinear version of the Fourier transform.
The Hirota bilinear method is the other direct but powerful technique to soliton waves. Hirota bilinear forms are the starting point to generate closed-form solutions [7,8]. In the (2+1)-dimensional case, take a polynomial
R in time
t and two space variables
. A (2+1)-dimensional Hirota bilinear differential equation is defined by
where
and
are three Hirota bilinear derivatives given as follows [2]:
in which
are nonnegative integers. Associated with a Hirota bilinear equation, a nonlinear partial differential equation
with a dependent variable
u is usually presented by the logarithmic derivative transformations
For multi-component integrable models (see, e.g., [5,6]), other kinds of transformations need to be introduced and implemented. Within the Hirota bilinear theory, an
N-soliton solution to a nonlinear equation can be presented by solving its corresponding Hirota bilinear equation (see, e.g., [7]-[11]).
Remarkably similar to solitons, lump waves (and rogue waves) are another kind of closed-form solutions to nonlinear integrable models [12]. Lump waves are expressed in terms of analytic rational functions, which are localized in all directions in the spatial space (see, e.g., [12,13]):
where
are arbitrary constants and
. The KPI equation possesses abundant lump waves (see, e.g., [8]), and taking long wave limits of its soliton solutions can yield particular lump waves [14]. Lump waves can exist in nonlinear nonintegrable models as well, and illustrative examples include generalized KP, BKP, Jimbo-Miwa and Bogoyavlensky-Konopelchenko equations [15]-[19]. There also exist lump waves in linear models in higher dimensions (see, e.g., [20]).
Quadratic functions are used to present exact solutions to Hirota bilinear equations and formulate lump wave solutions to nonlinear model equations [8,12]. The logarithmic derivative transformations are taken to link nonlinear model eqautions to bilinear equations. In this paper, we would like to search for lump waves in a spatial symmetric (2+1)-dimensional nonlinear dispersive wave model via such an ansatz using quadratic functions. The proposed spatial symmetric (2+1)-dimensional dispersive wave model contains three nonlinear terms. We will conduct symbolic computations with Maple to determine its lump waves. Characteristic properties, such as critical points and extreme values, will be analyzed for the resulting lump waves. Concluding remarks will be given in the last section.
2. A
spatial symmetric
nonlinear
model
and its Hirota bilinear form
Let
and
be real constants. We introduce a spatial symmetric (2+1)-dimensional nonlinear dispersive wave model equation:
with
, and search for its lump waves via the indicated ansatz using quadratic functions. The example with
ad
of this nonlinear model gives the special spatial symmetric (2+1)-dimensional model equation
Under the help fo Maple, through the logarithmic derivative transformations
the above spatial symmetric (2+1)-dimensional model equation (2.1) is put into the Hirota bilinear form:
where
and
are the standard Hirota bilinear derivatives [2] (see also, (1.2)). By symbolic computation, a precise relation between the nonlinear model equation and the bilinear model equation can be explored to be
where the involved functions
are determined through the logarithmic derivative transformations of
f in (2.3).
The same link also exists in a spatial symmetric KP model [21] and a spatial symmetric HSI model [22]. It is now evident that if f is a solution to the bilinear model equation (2.4), then determined by (2.3) solve the spatial symmetric (2+1)-dimensional dispersive wave model equation (2.1). In the following section, we would like to look for a class of lump waves in this spatial symmetric nonlinear dispersive wave model.
3. Lump wave solutions
We would now like to compute lump wave solutions to the spatial symmetric (2+1)-dimensional dispersive wave model equation (2.1), through conducting symbolic computations. A direct computation can show that the above general nonlinear model equation does not pass the three-soliton test (see, e.g., [9,11] for the three-soliton test) and thus it doesn’t possess an N-soliton solution..
Applying a general ansatz on lump waves in (2+1)-dimensions [8], we start looking for positive quadratic function solutions
to the corresponding Hirota bilinear equation (2.4), and the task will be to determine the real constant parameters
(see, e.g., [12,15,17] for illustrative examples). It is known that this is a general ansatz for lump wave solutions of lower order in (2+1)-dimensions [12].
We substitute
f by (3.1) into the Hirota bilinear equation (2.4) and obtain a system of algebraic equations on the involved parameters. A direct Maple computation to solve this system for
and
yields a set of solutions for the parameters:
and all other parameters are arbitrary.
The above two frequency parameters, and , exhibit a class of dispersion relations in (2+1)-dimensional nonlinear dispersive waves, and the constant term parameter, , tells a complicated expresssion of the wave numbers, which is crucial in formulating lump waves within the Hirota bilinear theory. Interestingly, there also exists a kind of higher-order dispersion relations appearing in lump waves of the second model equation of the integrable KP hierarchy [23].
Let us anlayze the analycity of the lump waves by observing the above simplified expressions for the wave frequencies and the constant term in (3.2). Obviously, if
then
This implies that if
, then we have
Therefore, to generate lump wave solutions through the logarithmic derivative transformations, we require two basic conditions:
and
Those two necessary and sufficient conditions really guarantee the fundamental properties of lump waves. First, the resulting solutions of
are localized in all spatial directions, under (3.5). Second, they are analytic in the whole spatial and temporal space, under (3.5) and (3.6), which lead equivalently to that
. We will show in the next section that
is also necessary for
to be analytic in
.
The second condition defined by (3.6) caontains the two coefficients,
and
. Clearly, if
then we have
. Therefore, the nonlinearity affects the analyricity of the lump waves in the model equation (2.1), but it does not affect the speeds of the two single waves in the lumps, in view of (3.2).
One reduced case can be worked out. When
and
, we obtain
Then, the conditions for the existence of lump waves in this reduced case simply become
4. Characteristics of the lump waves
In this section, we would like to consider the characteristic behaviors of the resultant lump waves presented previously.
4.1. Line of critical points
Let us first compute critical points of
f defined by (3.1) as a function of
x and
y. To this end, we need to determine solutions to the system
Since
f is a quadratic polynomial in
x and
y, this system just requires
Accordingly, based on the condition (3.5), we have
, i.e.,
This is a linear system of
x and
y, and all solutions are critical points of the quadratic function
f:
at an arbitrary time
t.
Evidently, those critical points form a straight line, whose two spatial coordinates travel at constant speeds. Now, a further straightforward computation can verify that all those points determined above are also critical points of the three solution functions and w defined by (2.3).
4.2. Analyticity condition
Taking advantage of (4.2), we see that the sum of two squares, i.e., the function becomes zero at all critical points defined by (4.3). Accordingly, the quadratic function in if and only if the constant term . The sufficiency is clear, as analyzed earlier. The necessity is true, because we have that f vanishes at the critical points if , and f vanishes at all pointa on the circle if .
Consequently, the three solutions defined by (2.3) are analytic in if and only if the constant parameter must be positive. Further, in view of the analysis on the positiveness of made in the previous section, the necessary and sufficient conditions for to be analytic are the two conditions in (3.5) and (3.6) on the wave numbers and the coefficients and .
4.3. Extreme values
Applying the second partial derivative test, we can see that the both lump waves,
v and
w, have a peak at the critical points
. This is because we have
and
where
is defined by
In a similar way, we can work out that
where
is given by (4.6). Accordingly, the lump wave
u has the maximum (or minimum) points
, when
(or
) and
the lump wave
u has the saddle points
, when
and the second partial derivative test is inconclusive, when
A direct computation can generate the extreme values of
and
u, achieved at the critical points
, as follows:
where
is defined by (4.6). Upon observing those expressions for the extreme values, we find that all extreme values do not depend on time
t; they are all constants on the characteristic line of critical points (see also, [21,22] for other examples). Furthermore, when
goes to zero, i.e., the two spatial directions
and
tends to be parallel to each other, the lump waves of
may not decay in all cases of the wave numbers
and
.
5. Conclusion
Through conducting symbolic computations with Maple, we have explored lump waves in a spatial symmetric (2+1)-dimensional dispersive wave model. The resulting lump waves have a line of critical points, whose spatial coordinates travel with constant velocities. The frequencies and the constant term of the lump waves were computed in terms of the wave numbers in the quadratic function f. Characteristic properties of the lump waves, such as critical points and extreme values, were worked out, and the effects of the nonlinear terms and the wave numbers were analyzed.
Interestingly, abundant lump waves also exist in linear wave model equations [20], besides nonlinear (2+1)-dimensional models (see, e.g., [24]-[27]) and (3+1)-dimension models (see, e.g., [28,29]). The Hirota bilinear forms and the generalized bilinear forms are the starting points [12,30], exhibiting a great convenience in determining lump waves. Interaction solutions between lump waves and other interesting waves, including homoclinic and heteroclinic solutions, can be explored for (2+1)-dimensonal integrable model equations (see, e.g., [16,31,32]).
It is also known that N-soliton solutions have been systematically studied by the Riemann-Hilbert technique for local and nonlocal integrable equations generated from groups reductions of matrix spectral problems (see, e.g., [33]-[36]). It is intriguing to analyze the existence of lump waves in reduced integarble equations (see, e.g., [37,38]), both local and nonlocal. It is expected that studies of lump waves could advance our understanding of nonlinear wave phenomena and their integrability theory [39].
Acknowledgments
The work was supported in part by NSFC under the grants 12271488, 11975145, 11972291 and 51771083, the Ministry of Science and Technology of China (G2021016032L), and the Natural Science Foundation for Colleges and Universities in Jiangsu Province (17 KJB 110020).
Data Availability Statement
All data generated or analyzed during this study are included in this published article.
Conflicts of Interest
The author declares that there is no known competing interest that could have appeared to influence this work.
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