Which theorem from Bohr's thesis was later independently derived by Hendrika van Leeuwen?

The theorem first obtained in Bohr’s thesis and later independently derived by Hendrika van Leeuwen is the Bohr–Van Leeuwen theorem.

Bohr reached the result in his 1911 doctoral dissertation while examining the classical electron theory of magnetism. Van Leeuwen independently rediscovered and developed the result in her 1919 doctoral thesis under Hendrik Lorentz at Leiden University; she later published work based on that thesis in 1921.

The theorem states that classical statistical mechanics predicts zero thermal magnetization for electrons in equilibrium, even in an applied magnetic field. Consequently, classical physics cannot explain ordinary magnetic properties of matter, including paramagnetism, diamagnetism, and ferromagnetism. Quantum mechanics is required to account for them.

The naming history is slightly unusual. In 1932, J. H. Van Vleck called it “Miss van Leeuwen’s theorem” while acknowledging Bohr’s earlier work. Modern usage combines both names. The other options concern symmetry, quantum nonlocality, or thermodynamic relations and are unrelated to this classical theory of magnetism.