A000099 counts lattice points inside expanding circles and links simple geometry to deep number theory. We explore how nr, the number of lattice points with x^2+y^2 ≤ R^2, relates to the circle’s area πR^2 via the error term E(R) = nr − πR^2; Gauss’s early bounds, Hardy–Landau lower bounds, and the current best upper bounds (due to Huxley) illustrate the subtle growth of the error. We touch on exact floor-function formulas, Bessel-function identities, and Jacobi’s two-squares connections, then move to higher dimensions with k-dimensional analogues (l_kx, a_kx, v_kx, p_kx) and the brave new world of octahedral spheres under L1 distance. The episode also highlights related problems like Dirichlet’s divisor problem and notable numerical experiments that shape our intuition about the true size of the error term. A000099 reveals the beautiful interplay between counting, geometry, and analytic methods in geometric number theory.
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