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Abstract

We present an unfitted boundary algebraic equation (BAE) method for solving elliptic partial differential equations in complex geometries. The method employs lattice Green's functions on infinite regular grids combined with discrete potential theory to construct single and double layer potentials, which is a discrete analog to boundary integral method. Local basis functions on cut cells accommodate arbitrary boundary conditions and seamlessly integrate with the boundary algebraic equations. The difference potentials framework enables efficient treatment of nonhomogeneous terms and fast computation of layer potentials via FFT-based solvers. We establish theoretical stability and convergence through a novel interpolation operator framework. Key advantages of the developed method include: dimension reduction, geometric flexibility, mesh-independent conditioning, small-cut stability, and uniform treatment of smooth and non-smooth geometries. Numerical experiments validate accuracy and robustness across ellipses and diamonds with varying aspect ratios and sharp corners, and an application of potential flows in unbounded domains.