  Discrete amorphous materials are best described in terms of arbitrary
networks which can be embedded in three dimensional space. Investigating the
thermodynamic equilibrium as well as non-equilibrium behavior of such materials
around second order phase transitions call for special techniques.
  We set up a renormalization group scheme by expanding an arbitrary scalar
field living on the nodes of an arbitrary network, in terms of the eigenvectors
of the normalized graph Laplacian. The renormalization transformation involves,
as usual, the integration over the more "rapidly varying" components of the
field, corresponding to eigenvectors with larger eigenvalues, and then
rescaling. The critical exponents depend on the particular graph through the
spectral density of the eigenvalues.
