In magnetic field, the charged particle cannot gain or lose energy. Reminder: equal-energy lines in the simple 2-dim band model: from circles to squares.
The evaluation of closed curve motion. This is closed curve in k-space, can be spiral in x-space. Line, i.e. curve, which is a cut of Fermi surface by a plane perpendicular to the magnetic field direction. Use the semiclassic equations of motion. Time intervals, path intervals on the Fermi curve.

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