A Hardy-type inequality and some spectral characterizations for the Dirac–Coulomb operator
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We prove a sharp Hardy-type inequality for the Dirac operator. We exploit this inequality to obtain spectral properties of the Dirac operator perturbed with Hermitian matrix-valued potentials $\mathbf V$ of Coulomb type: we characterise its eigenvalues in terms of the Birman—Schwinger principle and we bound its discrete spectrum from below, showing that the ground-state energy is reached if and only if $\mathbf V$ verifies some rigidity conditions. In the particular case of an electrostatic potential, these imply that $\mathbf V$ is the Coulomb potential.