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We consider the asymptotic analysis of the resonances of the scalar Helmholtz equation corresponding to a bounded scatterer with a periodic index of refraction and small period size ϵ. When the homogenized resonance is simple, we derive an explicit formula for the first order corrections to the limiting resonances. For scatterers with boundary that has a flat part of rational normal, the first order corrections are not unique and depend on the interaction of the boundary of the scatterer with the microstructure. In this case the resonances converge only O(ϵ) in general. For smooth domains with no flat parts, the resonances converge o(ϵ), but the convergence is nonetheless sub-quadratic. 

Abstract In previous work of the authors, we investigated the Born and inverse Born series for a scalar wave equation with linear and nonlinear terms, the nonlinearity being cubic of Kerr type (DeFilippiset al2023Inverse Problems39125015). We reported conditions which guarantee convergence of the inverse Born series, enabling recovery of the coefficients of the linear and nonlinear terms. In this work, we show that if the coefficient of the linear term is known, an arbitrarily strong Kerr nonlinearity can be reconstructed, for sufficiently small data. Additionally, we show that similar convergence results hold for general polynomial nonlinearities. Our results are illustrated with numerical examples. 

Data-driven reduced order models (ROMs) have recently emerged as an efcient tool for the solution of inverse scattering problems with applications to seismic and sonar imaging. One requirement of this approach is that it uses the full square multiple-input/multiple-output (MIMO) matrixvalued transfer function as the data for multidimensional problems. The synthetic aperture radar (SAR), however, is limited to the single-input/single-output (SISO) measurements corresponding to the diagonal of the matrix transfer function. Here we present a ROM-based Lippmann-Schwinger approach overcoming this drawback. The ROMs are constructed to match the data for each source-receiver pair separately, and these are used to construct internal solutions for the corresponding source using only the data-driven Gramian. Efficiency of the proposed approach is demonstrated on 2D and 2.5D (3D propagation and 2D reflectors) numerical examples. The new algorithm not only suppresses multiple echoes seen in the Born imaging but also takes advantage of their illumination of some back sides of the reflectors, improving the quality of their mapping. 

Abstract We consider the Born and inverse Born series for scalar waves with a cubic nonlinearity of Kerr type. We find a recursive formula for the operators in the Born series and prove their boundedness. This result gives conditions which guarantee convergence of the Born series, and subsequently yields conditions which guarantee convergence of the inverse Born series. We also use fixed point theory to give alternate explicit conditions for convergence of the Born series. We illustrate our results with numerical experiments. 

We present a survey of recent results on the inverse Born series. The convergence and stability of the method are characterized in Banach spaces. Applications to inverse problems in various physical settings are described.