The index of the spinc Dirac operator on the weighted projective space and the reciprocity law of the Fourier-Dedekind sum

The main purpose of this paper is to give a geometric interpretation of the reciprocity law of the Fourier-Dedekind sum given by M. Beck and S. Robins. In fact, the V-index of the spinc Dirac operator on the weighted projective space is equal to the dimension of the space of all weighted homogeneous polynomials of given degree, and this equality gives precisely the Beck-Robins reciprocity law. In this equality, the Fourier-Dedekind sums appear as the localization terms of the V-index of the spinc Dirac operators and have a relationship to the eta invariants of lens spaces.