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1. Optimizing Sparsity over Lattices and Semigroups

   https://doi.org/10.1007/978-3-030-45771-6_4  

   Aliev, I. (April 2020, Integer Programming and Combinatorial Optimization. IPCO 2020. Lecture Notes in Computer Science)

   Bienstock, D. (Ed.)

   Motivated by problems in optimization we study the sparsity of the solutions to systems of linear Diophantine equations and linear integer programs, i.e., the number of non-zero entries of a solution, which is often referred to as the ℓ0-norm. Our main results are improved bounds on the ℓ0-norm of sparse solutions to systems 𝐴𝑥=𝑏, where 𝐴∈ℤ𝑚×𝑛, 𝑏∈ℤ𝑚 and 𝑥 is either a general integer vector (lattice case) or a non-negative integer vector (semigroup case). In the lattice case and certain scenarios of the semigroup case, we give polynomial time algorithms for computing solutions with ℓ0-norm satisfying the obtained bounds. 

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2. Weak and strong solutions for a fluid‐poroelastic‐structure interaction via a semigroup approach

   https://doi.org/10.1002/mma.10533  

   Avalos, George; Gurvich, Elena; Webster, Justin_T (October 2024, Mathematical Methods in the Applied Sciences)

   A filtration system comprising a Biot poroelastic solid coupled to an incompressible Stokes free‐flow is considered in 3D. Across the flat 2D interface, the Beavers‐Joseph‐Saffman coupling conditions are taken. In the inertial, linear, and non‐degenerate case, the hyperbolic‐parabolic coupled problem is posed through a dynamics operator on a chosen energy space, adapted from Stokes‐Lamé coupled dynamics. A semigroup approach is utilized to circumvent issues associated to mismatched trace regularities at the interface. The generation of a strongly continuous semigroup for the dynamics operator is obtained via a non‐standard maximality argument. The latter employs a mixed‐variational formulation in order to invoke the Babuška‐Brezzi theorem. The Lumer‐Philips theorem then yields semigroup generation, and thereby, strong and generalized solutions are obtained. For the linear dynamics, density obtains the existence of weak solutions; we extend to the case where the Biot compressibility of constituents degenerates. Thus, for the inertial linear Biot‐Stokes filtration system, we provide a clear elucidation of strong solutions and a construction of weak solutions, as well as their regularity through associated estimates. 

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3. A comparative study of turbulent stratified shear layers: effect of density gradient distribution

   https://doi.org/10.1007/s10652-022-09873-2  

   Pham, Hieu T.; Sarkar, Sutanu (May 2022, Environmental Fluid Mechanics)

   Abstract Direct numerical simulations are performed to compare the evolution of turbulent stratified shear layers with different density gradient profiles at a high Reynolds number. The density profiles include uniform stratification, two-layer hyperbolic tangent profile and a composite of these two profiles. All profiles have the same initial bulk Richardson number ( $$Ri_{b,0}$$ R i b , 0 ); however, the minimum gradient Richardson number and the distribution of density gradient across the shear layer are varied among the cases. The objective of the study is to provide a comparative analysis of the evolution of the shear layers in term of shear layer growth, turbulent kinetic energy as well as the mixing efficiency and its parameterization. The evolution of the shear layers in all cases shows the development of Kelvin–Helmholtz billows, the transition to turbulence by secondary instabilities followed by the decay of turbulence. Comparison among the cases reveals that the amount of turbulent mixing varies with the density gradient distribution inside the shear layer. The minimum gradient Richardson number and the initial bulk Richardson number do not correlate well with the integrated TKE production, dissipation and buoyancy flux. The bulk mixing efficiency for fixed $$Ri_{b,0}$$ R i b , 0 is found to be highest in the case with two-layer density profile and lowest in the case with uniform stratification. However, the parameterizations of the flux coefficient based on buoyancy Reynolds number and the ratio of Ozmidov and Ellison scales show similar scaling in all cases. 

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4. A Remark on the Uniqueness of Solutions to Hyperbolic Conservation Laws

   https://doi.org/10.1007/s00205-023-01936-y  

   Bressan, Alberto; De Lellis, Camillo (December 2023, Archive for Rational Mechanics and Analysis)

   Given a strictly hyperbolic $$n\times n$$ system of conservation laws, it is well known that there exists a unique Lipschitz semigroup of weak solutions, defined on a domain of functions with small total variation, which are limits of vanishing viscosity approximations. The aim of this note is to prove that every weak solution taking values in the domain of the semigroup, and whose shocks satisfy the Liu admissibility conditions, actually coincides with a semigroup trajectory. 

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5. A min-plus fundamental solution semigroup for a class of approximate infinite dimensional optimal control problems

   McEneaney, William M; Dower, Peter M. (July 2020, Proceedings of the American Control Conference)

   By exploiting min-plus linearity, semiconcavity, and semigroup properties of dynamic programming, a fundamental solution semigroup for a class of approximate finite horizon linear infinite dimensional optimal control problems is constructed. Elements of this fundamental solution semigroup are parameterized by the time horizon, and can be used to approximate the solution of the corresponding finite horizon optimal control problem for any terminal cost. They can also be composed to compute approximations on longer horizons. The value function approximation provided takes the form of a min-plus convolution of a kernel with the terminal cost. A general construction for this kernel is provided, along with a spectral representation for a restricted class of sub-problems. 

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