Rotation Numbers, Boundary Forces and Gap labelling
Abstract
We review the Johnson-Moser rotation number and the K0-theoretical gap labelling of Bellissard for one-dimensional Schr\"odinger operators. We compare them with two further gap-labels, one being related to the motion of Dirichlet eigenvalues, the other being a K1-theoretical gap label. We argue that the latter provides a natural generalisation of the Johnson-Moser rotation number to higher dimensions.
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