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1. Small-world disordered lattices: spectral gaps and diffusive transport

   https://doi.org/10.1088/1367-2630/ac7db5  

   Rosa, Matheus I. N.; Ruzzene, Massimo (July 2022, New Journal of Physics)

   Abstract We investigate the dynamic behavior of lattices with disorder introduced through non-local network connections. Inspired by the Watts–Strogatz small-world model, we employ a single parameter to determine the probability of local connections being re-wired, and to induce transitions between regular and disordered lattices. These connections are added as non-local springs to underlying periodic one-dimensional (1D) and two-dimensional (2D) square, triangular and hexagonal lattices. Eigenmode computations illustrate the emergence of spectral gaps in various representative lattices for increasing degrees of disorder. These gaps manifest themselves as frequency ranges where the modal density goes to zero, or that are populated only by localized modes. In both cases, we observe low transmission levels of vibrations across the lattice. Overall, we find that these gaps are more pronounced for lattice topologies with lower connectivity, such as the 1D lattice or the 2D hexagonal lattice. We then illustrate that the disordered lattices undergo transitions from ballistic to super-diffusive or diffusive transport for increasing levels of disorder. These properties, illustrated through numerical simulations, unveil the potential for disorder in the form of non-local connections to enable additional functionalities for metamaterials. These include the occurrence of disorder-induced spectral gaps, which is relevant to frequency filtering devices, as well as the possibility to induce diffusive-type transport which does not occur in regular periodic materials, and that may find applications in dynamic stress mitigation. 

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2. A manifold two-sample test study: integral probability metric with neural networks

   https://doi.org/10.1093/imaiai/iaad018  

   Wang, Jie; Chen, Minshuo; Zhao, Tuo; Liao, Wenjing; Xie, Yao (June 2023, Information and Inference: A Journal of the IMA)

   Abstract Two-sample tests are important areas aiming to determine whether two collections of observations follow the same distribution or not. We propose two-sample tests based on integral probability metric (IPM) for high-dimensional samples supported on a low-dimensional manifold. We characterize the properties of proposed tests with respect to the number of samples $$n$$ and the structure of the manifold with intrinsic dimension $$d$$. When an atlas is given, we propose a two-step test to identify the difference between general distributions, which achieves the type-II risk in the order of $$n^{-1/\max \{d,2\}}$$. When an atlas is not given, we propose Hölder IPM test that applies for data distributions with $$(s,\beta )$$-Hölder densities, which achieves the type-II risk in the order of $$n^{-(s+\beta )/d}$$. To mitigate the heavy computation burden of evaluating the Hölder IPM, we approximate the Hölder function class using neural networks. Based on the approximation theory of neural networks, we show that the neural network IPM test has the type-II risk in the order of $$n^{-(s+\beta )/d}$$, which is in the same order of the type-II risk as the Hölder IPM test. Our proposed tests are adaptive to low-dimensional geometric structure because their performance crucially depends on the intrinsic dimension instead of the data dimension. 

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3. On the well-posedness of the periodic fractional Schrödinger equation

   https://doi.org/10.1007/s42985-025-00327-0  

   Sanchez, Beckett; Riaño, Oscar; Roudenko, Svetlana (April 2025, Partial Differential Equations and Applications)

   We consider the periodic fractional nonlinear Schroedinger equation, where the nonlinearity is expressed in two ways either as a derivative of a polynomial (including negative powers, such including logarithmic nonlinearities), or given as a sum of powers, possibly infinite. We obtain the local well-posedness for the Cauchy problem of this equation in weighted Sobolev spaces and with non-vanishing initial data. 

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4. Reduced models for electron magnetohydrodynamics: Well-posedness and singularity formation

   https://doi.org/10.1090/tran/9427  

   Dai, Mimi (June 2025, Transactions of the American Mathematical Society)

   We propose one-dimensional reduced models for the three-dimensional electron magnetohydrodynamics which involves a highly nonlinear Hall term with intricate structure. The models contain nonlocal nonlinear terms which are more singular than that of the one-dimensional models for the Euler equation and the surface quasi-geostrophic equation. Local well-posedness is obtained in certain circumstances. Moreover, for a model with nonlocal transport term, we show that singularity develops in finite time for a class of initial data. 

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5. Well-posedness in weighted spaces for the generalized Hartree equation with p < 2

   https://doi.org/10.1142/S0219199721500747  

   Arora, Anudeep K.; Riaño, Oscar; Roudenko, Svetlana (November 2022, Communications in Contemporary Mathematics)

   We investigate the well-posedness in the generalized Hartree equation [Formula: see text], [Formula: see text], [Formula: see text], for low powers of nonlinearity, [Formula: see text]. We establish the local well-posedness for a class of data in weighted Sobolev spaces, following ideas of Cazenave and Naumkin, Local existence, global existence, and scattering for the nonlinear Schrödinger equation, Comm. Contemp. Math. 19(2) (2017) 1650038. This crucially relies on the boundedness of the Riesz transform in weighted Lebesgue spaces. As a consequence, we obtain a class of data that exists globally, moreover, scatters in positive time. Furthermore, in the focusing case in the [Formula: see text]-supercritical setting we obtain a subset of locally well-posed data with positive energy, which blows up in finite time. 

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