We prove the existence of solitary waves in the KdV limit of two-dimensional FPU-type lattices using asymptotic analysis of nonlinear and singularly perturbed integral equations. In particular, we generalize the existing results by Friesecke and Matthies since we allow for arbitrary propagation directions and non-unidirectional wave profiles.
Keywords:two-dimensional FPU-lattices, KdV limit of lattice waves, asymptotic analysis of singularly perturbed integral equations MSC (2010): 37K60, 37K40, 74H10in [15] to 1D lattices with nonlocal interactions, replacing the original analysis of Fourier poles in the complex plane by direct estimates on the real line. The KdV approximation can also be used to derive modulation equations for the large-scale dynamics of well-chosen initial data. We refer to [23,17,18,19] for results in the classical FPU setting, and to [12] for more complex atomic systems.Existence results on various types of waves in FPU chains can be found in the literature. The paper [21] provides a precise description of all subsonic and supersonic waves with small amplitudes via the center manifold reduction. In [11] the existence of solitary waves with large amplitudes was proved for the first time by applying the concentration-compactness principle, see also [1] for similar results under slightly different conditions and [4,13] for another variational approach yielding unimodal waves. In all these large-amplitude results the wave speed is in principle unknown. However, using mountain pass techniques as in [25,22] one also finds periodic and solitary waves with prescribed wave speed above a critical threshold. Finally, for certain potentials there even exist several types of heteroclinic waves, see [16,26,24].The investigations mentioned above concern exclusively 1D FPU lattices. In the practice of physics, two or more dimensional FPU-lattices are more relevant, since they provide simplified models for crystals and solids. However, to the best of our knowledge, no existence result for KdV-like waves in such FPU lattices has been obtained except for a degenerate case [5], where it is assumed that the wave is unidirectional and longitudinal as it moves along the horizontal direction. This assumption reduces the original two-dimensional system to a one-dimensional one and the geometric nonlinearity, which arises from the linear stress-strain relation due to the introduction of diagonal springs, enables one to apply the results from [7].In the present paper we show an existence result for 2D FPU lattices in a general framework, which covers different lattice geometries and allows for arbitrary propagation directions. Our asymptotic approach adapts some arguments developed in [15] but is more sophisticated due to the two-dimensional setting. The discussion of the 2D case in the present paper hints at similar results for 3D lattices, which have to be left for further investigations. Besides, the study of stability of the resulting solutions requires ideas lying beyond the scope of the present paper and will be...