Soliton molecules and mixed solutions of the (2【-逻*辑*与-】plus;1)-dimensional bidirectional Sawada【-逻*

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Soliton molecules and mixed solutions of the (2【-逻辑与-】plus;1)-dimensional bidirectional Sawada【-逻*

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Jiao-Jiao Dong¹, Biao Li^,¹, Manwai Yuen²¹School of Mathematics and Statistics, Ningbo University, Ningbo 315211, China
²Department of Mathematics and Information Technology, The Education University of Hong Kong, Lo Ping Road, Tai Po, New Territories, Hong Kong, China

Received:2019-10-14Revised:2019-11-5Accepted:2019-12-8Online:2020-01-29

Abstract
In this paper, we investigate some new interesting solution structures of the (2+1)-dimensional bidirectional Sawada–Kotera (bSK) equation. We obtain soliton molecules by introducing velocity resonance. On the basis of soliton molecules, asymmetric solitons are obtained by changing the distance between two solitons of molecules. Based on the N-soliton solutions, several novel types of mixed solutions are generated, which include the mixed breather-soliton molecule solution by the module resonance of the wave number and partial velocity resonance, the mixed lump-soliton molecule solution obtained by partial velocity resonance and partial long wave limits, and the mixed solutions composed of soliton molecules (asymmetric solitons), lump waves, and breather waves. Some plots are presented to clearly illustrate the dynamic features of these solutions.
Keywords： (2+1)-dimensional bSK equation;velocity resonance;soliton molecules;asymmetric solitons;mixed solutions

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Jiao-Jiao Dong, Biao Li, Manwai Yuen. Soliton molecules and mixed solutions of the (2+1)-dimensional bidirectional Sawada–Kotera equation. Communications in Theoretical Physics, 2020, 72(2): 025002- doi:10.1088/1572-9494/ab6184

1. Introduction

Recently, the study of soliton molecules has attracted increasing attention. Soliton molecules, called a soliton complex, were proposed by Akhmediev in 2000 [1]. Soliton molecules are a bound state consisting of multiple solitons in the optical cavity at the same time [2–5]. It has been observed experimentally in optics [2, 4, 6]. Solitons, particle-like excitations ubiquitous in many fields of physics, have been shown to exhibit bound states akin to molecules, such as plasmas physics [7], optics [8], fluid dynamics [9], and Bose–Einstein condensates [10] and complex networks [11]. The formation of such temporal soliton bound states and their internal dynamics have been observed with difficulty using direct experimental observation. And research has shown that real-time detection in resolving interactions in complex nonlinear systems is of great importance, and includes the dynamics of soliton bound states, breathers, and rogue waves.

As we all know, exact solutions for the nonlinear evolution equations (NLEEs) have been applied in the nonlinear science fields. Rational solutions of many NLEEs have been investigated by researchers [12–17]. Among these rational solutions, lump solutions, breather wave solutions, and rogue wave solutions are hot points all the time. Moreover, the mixed solution is not just specific for the (2+1)-dimensional NLEEs. For the much simpler (1+1)-dimensional NLEEs, such a solution is not unusual in the vector counterparts [18–21]. The mixed solutions consisting of lump solutions with other types of solutions attract a lot of attention, and include lump-solitons [22], lump-kink solutions [23], and resonance stripe solitons [13, 24, 25]. More similar mixed solutions have been analyzed [26–28]. For breather interaction in the NLEEs, super-regular breathers are also a hot point. A great deal of work has been carried out [29–33]. In [34], Lou gave the soliton molecules and asymmetric solitons of three (1+1)-dimensional integrable cases via velocity resonance, which are the fifth order Korteweg–de Vries (KdV) equation, the Sawada–Kotera (SK) equation and the Kaup–Kupershmidt (KK) equation. Thus, we start to think about other integrable equations and try to find the soliton molecules, asymmetric solitons, and the new types of mixed solutions, such as the mixed breather-soliton molecule solution, the mixed lump-soliton molecule solution, and the mixed solution composed of lump waves, breather waves, and soliton molecules (asymmetric solitons). In this paper, we focus on a (2 + 1)-dimensional bidirectional SK (bSK) equation [35](1)$\begin{eqnarray}\begin{array}{l}-45{u}^{2}{u}_{x}-15{{uu}}_{{xxx}}-15{u}_{x}{u}_{{xx}}-15{{uu}}_{t}-15{u}_{x}{\left({\partial }_{x}\right)}^{-1}({u}_{t})\\ \quad +\,5{\left({\partial }_{x}\right)}^{-1}{u}_{{tt}}-5{u}_{{xxt}}-{u}_{{xxxxx}}+9{u}_{y}=0,\end{array}\end{eqnarray}$where(2)$\begin{eqnarray}\begin{array}{l}{\left({\partial }_{x}\right)}^{-n}={\left(\displaystyle \frac{d}{{d}_{x}}\right)}^{-n}.\end{array}\end{eqnarray}$It was formulated there as a bidirectional generalization of the SK equation(3)$\begin{eqnarray}\begin{array}{l}{u}_{t}+45{u}^{2}{u}_{x}-15{u}_{x}{u}_{{xx}}-15{{uu}}_{{xxx}}+{u}_{{xxxxx}}=0.\end{array}\end{eqnarray}$The SK equation is an important unidirectional nonlinear evolution equation. Its mathematical properties are well documented in the literature [36–40]. The authors pointed out its bidirectional nature and relation to the SK equation (2) through the normal form of equation (1) in [41, 42]. Moreover, it is often preferable in many physical situations to have an equation which allows us to model waves that propagate in opposite directions. Therefore, the study of a bSK equation is quite interesting. Some work has been carried out on investigation of the bSK equation (1). For instance, Darboux and Bäklund transformations of the bSK equation (1) are derived, and some approximate and exact solutions of the bSK equation (1) are obtained by means of Maple, such as solitary wave solutions, doubly periodic solutions, and two-soliton solutions [43, 44]. However, to the best of the authors’ knowledge, soliton molecules, asymmetric solitons and the mixed solutions between breather waves (lump waves) and soliton molecules (asymmetric solitons), and the mixed solutions composed of lump waves, breather waves, and soliton molecules (asymmetric solitons) for the bSK equation (1) have not been investigated before.

Motivated by the goal of making further progress with the bSK equation, we introduce a new possibility, the velocity resonant mechanism, to form soliton molecules of the (2 + 1)-dimensional bSK equation. Under the resonant mechanism, asymmetric solitons can be generated by changing the distance between two solitons of molecules. And we can obtain the mixed breather-soliton molecule solution by the module resonance of the wave number and partial velocity resonance, and the mixed lump-soliton molecule solution by partial velocity resonance and partial long wave limits. The rest of the paper is organized as follows: in section 2, we give soliton molecules and asymmetric solitons of the bSK equation. In sections 3 and 4, based on the N-soliton solution, the mixed breather-soliton molecule solution and the mixed lump-soliton molecule solution are yielded. In section 5, we obtain the mixed solutions composed of a lump wave, a breather wave, and a soliton molecule (asymmetric solitons). And the general case for the mixed solution is given. Our results are summarised in the final section.

2. Soliton molecules and asymmetric solitons of the (2 + 1)-D bSK equation

In this section, we focus on soliton molecules of the (2 + 1)-dimensional bSK equation (1). The Hirota bilinear method in soliton theory provides a powerful approach to looking for exact solutions. Within the Hirota bilinear formulation, solitons can be usually generated as follows(4)$\begin{eqnarray}\begin{array}{rcl}u & = & 2{\left(\mathrm{ln}f\right)}_{{xx}},\\ f & = & \displaystyle \sum _{\mu =0,1}\exp \left(\displaystyle \sum _{i=1}^{N}{\mu }_{i}{\xi }_{i}+\displaystyle \sum _{i\lt j}^{N}{\mu }_{i}{\mu }_{j}{A}_{{ij}}\right),\end{array}\end{eqnarray}$where(5)$\begin{eqnarray}\begin{array}{l}u=u(x,y,t),\\ {\xi }_{i}={k}_{i}x+{p}_{i}y+{w}_{i}t+{\phi }_{i},1\lt i\lt N,\\ 9{p}_{s}{k}_{s}-{k}_{s}^{6}-5{k}_{s}^{3}{w}_{s}+5{w}_{s}^{2}=0,\end{array}\end{eqnarray}$and(6)$\begin{eqnarray}\begin{array}{l}\exp ({A}_{{ij}})=-\displaystyle \frac{{\left({k}_{i}-{k}_{j}\right)}^{6}+5{\left({k}_{i}-{k}_{j}\right)}^{3}({w}_{i}-{w}_{j})-9({p}_{i}-{p}_{j})({k}_{i}-{k}_{j})-5{\left({w}_{i}-{w}_{j}\right)}^{2}}{{\left({k}_{i}+{k}_{j}\right)}^{6}+5{\left({k}_{i}+{k}_{j}\right)}^{3}({w}_{i}+{w}_{j})-9({p}_{i}+{p}_{j})({k}_{i}+{k}_{j})-5{\left({w}_{i}+{w}_{j}\right)}^{2}}.\end{array}\end{eqnarray}$To find nonsingular analytical resonant excitations from u in equation (4), we introduce a novel type of resonant conditions:(1) ${k}_{i}\ne \pm {k}_{j},{p}_{i}\ne \pm {p}_{j};$
(2) the velocity resonance,(7)$\begin{eqnarray}\displaystyle \frac{{k}_{i}}{{k}_{j}}=\displaystyle \frac{{w}_{i}}{{w}_{j}}=\displaystyle \frac{{p}_{i}}{{p}_{j}}.\end{eqnarray}$Under the resonant conditions, we can select appropriate solution parameters, which is certain to cause solitons to form a soliton molecule or an asymmetric soliton through calculation. Next, we take N=2 as an example to see this fact. To exhibit one soliton molecule structure, a two-soliton solution needs to satisfy the following resonance condition: the velocity resonance is(8)$\begin{eqnarray}{k}_{2}=\sqrt{-\displaystyle \frac{{k}_{1}^{3}+5{w}_{1}}{{k}_{1}}},{w}_{2}=\displaystyle \frac{\sqrt{-\tfrac{{k}_{1}^{3}+5{w}_{1}}{{k}_{1}}}{w}_{1}}{{k}_{1}}\end{eqnarray}$or(9)$\begin{eqnarray}{k}_{2}=-\sqrt{-\displaystyle \frac{{k}_{1}^{3}+5{w}_{1}}{{k}_{1}}},{w}_{2}=-\displaystyle \frac{\sqrt{-\tfrac{{k}_{1}^{3}+5{w}_{1}}{{k}_{1}}}{w}_{1}}{{k}_{1}},\end{eqnarray}$and parameter selections are as follows:(10)$\begin{eqnarray}\begin{array}{rcl}{w}_{1} & = & \displaystyle \frac{19}{5},{k}_{1}=-\displaystyle \frac{5}{2},{k}_{2}=\displaystyle \frac{3\sqrt{3}\sqrt{5}}{10},\\ {w}_{2} & = & -\displaystyle \frac{57\sqrt{3}\sqrt{5}}{125},{\phi }_{1}=0,{\phi }_{2}=10.\end{array}\end{eqnarray}$

From figure 1, one can clearly understand the soliton molecule structure of the (2+1)-dimensional bSK equation, and find that there are obvious differences between two solitons in the molecule, although they possess identical velocities. Their height and width are visibly different. And it is interesting that parameter φ_i decides the distance. When they are close enough, the molecules look like one peak soliton, which is called an asymmetric soliton. Thus, an asymmetric one-soliton solution will be generated from the soliton molecule. We take the same parameters as those in figure 1, except for ${\phi }_{2}=\tfrac{41}{10}$. The plots of the asymmetric soliton are shown in figure 2.

Figure 1.

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Figure 1.(a): A three-dimensional plot of the soliton molecule of the (2+1)-dimensional bSK equation; and (b) the density plots of the soliton molecule structure.

Figure 2.

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Figure 2.(a): A three-dimensional plot of the asymmetric soliton of the (2+1)-dimensional bSK equation; (b) the density plots of the asymmetric soliton; and (c) the two-dimensional plot when x=0 at t=−5, 0, 5.

By comparing figure 1(b) with figure 2(b), we can directly ascertain that the wave width of the asymmetric soliton is wider than the soliton molecule. And from figure 1(a) with figure 2(a), we also find that the height of the asymmetric soliton is between the height of two solitons in the molecule.

As we all know, the interactions among solitons for the bSK equation are elastic. Therefore, we believe that the interactions among soliton molecules are also elastic. To illustrate this elastic phenomena between two soliton molecule solutions, we select suitable parameters for equation (4) and N=4:(11)$\begin{eqnarray}\begin{array}{rcl}{k}_{1} & = & \displaystyle \frac{4}{5},{k}_{2}=\displaystyle \frac{1\sqrt{2677}\sqrt{700}}{700},{k}_{3}=-\displaystyle \frac{1}{2},{k}_{4}=-\displaystyle \frac{\sqrt{37}\sqrt{12}}{12},\\ {w}_{1} & = & -\displaystyle \frac{5}{7},{w}_{2}=-\displaystyle \frac{\sqrt{2677}\sqrt{700}}{784},{w}_{3}=\displaystyle \frac{1}{3},{w}_{4}=\displaystyle \frac{\sqrt{37}\sqrt{12}}{18},\\ {\phi }_{1} & = & -10,{\phi }_{2}=10,{\phi }_{3}=-5,{\phi }_{4}=10.\end{array}\end{eqnarray}$

The plots of the elastic interaction properties for the two soliton molecule solution are shown in figure 3. From figure 3(a), we can see that the heights of the wave peaks and the velocities of the wave peaks remain unchanged.

Figure 3.

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Figure 3.(a): The density plots of the elastic interaction properties for two soliton molecules of the (2+1)-dimensional bSK equation; (b) the density plots of the elastic interaction properties for two asymmetric solitons with parameters (11), except for φ₂=−22, φ₃=3.

3. Mixed solutions between soliton molecules and breathers of the bSK equation

In this section, we will study the mixed solution between soliton molecules and breathers of the (2+1)-dimensional bSK equation. To obtain the mixed solution, we need the four-soliton solutions of equation (1), and the first two solitons must satisfy the velocity resonance; the other two solitons satisfy the module resonance of the wave numbers ${k}_{i}=\overline{{k}_{j}}$ and ${w}_{i}=\overline{{w}_{j}}$. We select the following suitable parameters in equation (4):(12)$\begin{eqnarray}\begin{array}{rcl}{k}_{1} & = & -\displaystyle \frac{5}{2},{k}_{2}=\displaystyle \frac{3\sqrt{3}\sqrt{5}}{10},{w}_{1}=\displaystyle \frac{19}{5},\\ {w}_{2} & = & -\displaystyle \frac{57\sqrt{3}\sqrt{5}}{125},{\phi }_{1}=0,{\phi }_{2}=35,\\ {k}_{3} & = & \displaystyle \frac{1}{4}-\displaystyle \frac{1}{2}{\rm{i}},{k}_{4}=\displaystyle \frac{1}{4}+\displaystyle \frac{1}{2}{\rm{i}},{w}_{3}=\displaystyle \frac{1}{3},\\ {w}_{4} & = & \displaystyle \frac{1}{3},{\phi }_{3}=0,{\phi }_{4}=0.\end{array}\end{eqnarray}$

The plots of the mixed solution between soliton molecules and breathers are shown in figure 4(a). As we can see, the interactions between soliton molecules and breathers are still elastic. We will naturally consider the mixed solution between the asymmetric soliton molecule and breathers. We know that the asymmetric soliton is generated from soliton molecules with identical parameter selections, except for φ_i. Thus, one can obtain the mixed solution between the asymmetric soliton molecule and breathers. Figure 4(c) displays the elastic interaction properties of the mixed solution between the asymmetric soliton molecule and breathers.

Figure 4.

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Figure 4.(a) A three-dimensional plot of a mixed solution between soliton molecules and breathers with the parameters in equation (12) at t=0; (c) a three-dimensional plot of a mixed breather-asymmetric soliton solution with the parameters in equation (12), except for φ₂=−3 at t=0; (b) and (d) density plots of the mixed breather-soliton molecule solution and the mixed lump-soliton molecule solution.

4. Mixed solutions between soliton molecules and lumps of the bSK equation

In this section, we will study a mixed solution between soliton molecules and lumps of the (2+1)-dimensional bSK equation. We can apply a partial velocity resonance and a partial long wave limit to the four-soliton solution of equation (1) to obtain the mixed solutions consisting of lump waves and soliton molecules or asymmetric solitons. To display the collision lump wave with soliton molecules, we take some suitable parameters for equation (4) as follows:(13)$\begin{eqnarray}\begin{array}{rcl}N & = & 4,{k}_{1}=-\displaystyle \frac{5}{2},{k}_{2}=\displaystyle \frac{3\sqrt{3}\sqrt{5}}{10},{w}_{1}=\displaystyle \frac{19}{5},\\ {w}_{2} & = & -\displaystyle \frac{57\sqrt{3}\sqrt{5}}{125},{\phi }_{1}=0,{\phi }_{2}=10,\\ {k}_{3} & = & (1-{\rm{i}})\epsilon ,{k}_{4}=(1+{\rm{i}})\epsilon ,\\ {w}_{3} & = & \displaystyle \frac{7\epsilon }{2},{w}_{4}=\displaystyle \frac{7\epsilon }{2},{\phi }_{3}={\rm{i}}\pi ,{\phi }_{4}={\rm{i}}\pi .\end{array}\end{eqnarray}$Then, we can obtain the mixed solution of equation (1) by taking a long wave limit $(\epsilon \to 0)$. Therefore, we can exhibit plots of the collision lump wave with the soliton molecules in figure 5. Similarly, we consider the mixed lump-asymmetric soliton solution; the parameter selection is the same as equation (13), except for φ₂. The plot is shown in figure 6.

Figure 5.

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Figure 5.Time evolution of the mixed lump-soliton molecule solution of equation (1) with the parameters in equation (5): three-dimensional plots with (a) t=−3, (b) t=1, and (c) t=5.

Figure 6.

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Figure 6.Time evolution of the mixed lump-asymmetric soliton solution of equation (1) with the parameters in equation (5), except for ${\phi }_{2}=-\tfrac{41}{10}$: three-dimensional plots with (a) t=−4, (b) t=−1, and (c) t=2.

5. Mixed solutions consisting of soliton molecules, breathers, and lumps of the bSK equation

In this section, we will introduce the mixed solutions consisting of soliton molecules (asymmetric solitons), breathers, and lumps of the bSK equation. Similarly, these mixed solutions still need to satisfy the resonant condition, and take the partial long wave limit.

More generally, we can take the following parameter constraints in equation (4) to obtain the mixed solutions composed of m soliton molecules, n breather waves, and q lump waves.(14)$\begin{eqnarray}\begin{array}{rcl} & & \displaystyle \frac{{k}_{1}}{{k}_{2}}=\displaystyle \frac{{w}_{1}}{{w}_{2}}=\displaystyle \frac{{p}_{1}}{{p}_{2}},\cdot \cdot \cdot ,\,\displaystyle \frac{{k}_{2m-1}}{{k}_{2m}}=\displaystyle \frac{{w}_{2m-1}}{{w}_{2m}}=\displaystyle \frac{{p}_{2m-1}}{{p}_{2m}},\\ & & {\xi }_{2m+1}=\overline{{\xi }_{2m+2}},\cdot \cdot \cdot ,\,{\xi }_{2m+2n-1}=\overline{{\xi }_{2m+2n}},\\ & & {k}_{2m+2n+1}={K}_{2m+2n+1}\epsilon ,{k}_{2m+2n+2}=\overline{{K}_{2m+2n+1}}\epsilon ,\cdot \cdot \cdot ,\\ & & {k}_{2m+2n+2q-1}={K}_{2m+2n+2q-1}\epsilon ,\\ & & {k}_{2m+2n+2q}=\overline{{K}_{2m+2n+2q-1}}\epsilon ,\\ & & {w}_{2m+2n+1}={W}_{2m+2n+1}\epsilon ,{w}_{2m+2n+2}=\overline{{W}_{2m+2n+1}}\epsilon ,\cdot \cdot \cdot ,\\ & & {w}_{2m+2n+2q-1}={W}_{2m+2n+2q-1}\epsilon ,\\ & & {w}_{2m+2n+2q}=\overline{{W}_{2m+2n+2q-1}}\epsilon ,\\ & & {\phi }_{2m+2n+1}=\pi {\rm{i}},{\phi }_{2m+2n+2}=\pi {\rm{i}},\cdot \cdot \cdot ,\,{\phi }_{2m+2n+2q}=\pi {\rm{i}}.\end{array}\end{eqnarray}$

To clearly describe the interaction phenomenon, we take a simple example for N=6. To obtain the mixed solution, k₁, k₂, w₁, and w₂ must satisfy the resonant condition, and k₅, k₆, w₅, and w₆ satisfy the conjuation condition. Then, we take a long wave limit on k₃, k₄, w₃, w₄ ($\epsilon \to 0$), the parameter selections as follows:(15)$\begin{eqnarray}\begin{array}{rcl}{k}_{1} & = & -\displaystyle \frac{5}{2},{k}_{2}=\displaystyle \frac{3\sqrt{3}\sqrt{5}}{10},{w}_{1}=\displaystyle \frac{19}{5},\\ {w}_{2} & = & -\displaystyle \frac{57\sqrt{3}\sqrt{5}}{125},{\phi }_{1}=0,{\phi }_{2}=35,\\ {k}_{3} & = & (2-{\rm{i}})\epsilon ,{k}_{4}=(2+{\rm{i}})\epsilon ,{w}_{3}=(9/2)\epsilon ,\\ {w}_{4} & = & (9/2)\epsilon ,{\phi }_{3}=\pi {\rm{i}},{\phi }_{4}=\pi {\rm{i}},\\ {k}_{5} & = & \displaystyle \frac{1}{4}-\displaystyle \frac{1}{2}{\rm{i}},{k}_{6}=\displaystyle \frac{1}{4}+\displaystyle \frac{1}{2}{\rm{i}},\\ {w}_{5} & = & \displaystyle \frac{1}{3},{w}_{6}=\displaystyle \frac{1}{3},{\phi }_{5}=0,{\phi }_{6}=0.\end{array}\end{eqnarray}$

Figure 7(a) displays the mixed solutions between a soliton molecule, a lump wave, and a breather wave described by equation (4). We can get the mixed solution consisting of a lump wave, a breather wave, and asymmetric solitons shown in figure 7(b) by changing the distance between the two solitons in the molecule. From the plots, we can find that this interaction phenomenon is quite interesting. We obtain the mixed solutions between a soliton molecule, a lump wave, and a breather wave with parameters in equation (15). As time goes on, we see that the distance between the two solitons in the molecule is growing closer. At the same time, we can obtain the mixed solution between an asymmetric soliton, a lump wave, and a breather wave with the parameter selections in equation (15), except for φ₂=−3. In other words, the phase position φ decides the interaction phenomenon. The interaction between these waves is also elastic.

Figure 7.

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Figure 7.(a) Elastic interaction properties between a soliton molecule, a lump wave, and a breather wave for equation (1) described by equation (4) with parameter selections from equation (15); (b) Elastic interaction properties between an asymmetric soliton, a lump wave, and a breather wave for equation (1) described by equation (4) with parameter selections from equation (15), except for φ₂=−3.

6. Conclusion

In this paper, we investigated soliton molecules, asymmetric solitons, and the mixed solutions between breather wave (lump wave) and soliton molecules (soliton molecules), and mixed solutions composed of soliton molecules (asymmetric solitons), lump waves, and breather waves of the (2+1)-dimensional bSK equation. And we have obtained the mixed breather-soliton molecule solution by the module resonance of the wave number and partial velocity resonance, and the mixed lump-soliton molecule solution by the partial velocity resonance and partial long wave limits. At the same time, we present three-dimensional plots and density plots of these solutions. In figure 1, it is shown that the two solitons in the molecule possess different heights and widths. Asymmetric solitons are shown in figure 2. Figure 3 indicates that the collision of two soliton molecules is elastic. The mixed solutions between soliton molecules (asymmetric soliton) and a breather wave are shown in figure 4. From figures 5 and 6, we can see the three-dimensional plots of the mixed solutions between soliton molecules (asymmetric soliton) and lump waves, respectively, at three different times. The mixed solutions consisting of soliton molecules (asymmetric solitons), lump waves, and breather waves are also obtained; see figure 7. We also give the general restrictions for obtaining these new mixed solutions containing m soliton molecules, n breather waves, and q lump waves. Finally, it is expected that these results can be useful in helping one to realize the dynamical behavior of nonlinear waves in fluid fields, and may provide some new perspectives to study the soliton molecules of nonlocal systems. Moreover, we will study super-regular breathers of the (2+1)-dimensional bSK equation in the future.

Acknowledgments

The authors would like to express their sincere thanks to Professor Lou Senyue for his guidance and encouragement. This work is supported by the National Natural Science Foundation of China (Nos. 11775121 and 11435005), the Dean’s Research Fund 2017-18 (FLASS/DRF/IRS-10) from the Education University of Hong Kong and the KC Wong Magna Fund at Ningbo University.