Model: a point particle in a central force F = k/r^n (n = d-1 for a flux-conserving force in d spatial dimensions; our space is n = 2). Leapfrog integration with adaptive sub-steps. Bertrand (1873): only n = 2 and n = -1 give closed orbits for all bound states; for n ≥ 3 no stable circular orbit exists (Ehrenfest 1917). Near-circular apsidal angle = π/√(3-n).