Gap labelling and the pressure on the boundary

In quantum systems described by covariant families of 1-particle Schrodinger operators on half-spaces the pressure on the boundary per unit energy is topologically quantised if the Fermi energy lies in a gap of the bulk spectrum. Its relation with the integrated density of states can be expressed in an integrated version of Streda's formula. This leads also to a gap labelling theorem for systems with constant magnetic field. The proof uses non-commutative topology.