  A close study is made of the static structure factor for graphene in a
magnetic field at integer filling factors nu, with focus on revealing possible
signatures of "relativistic" quantum field theory in the low-energy physics of
graphene. It is pointed out, in particular, that for graphene even the vacuum
state has a nonzero density spectral weight, which, together with the structure
factor for all nu, grows significantly with increasing wave vector; such
unusual features of density correlations are a "relativistic" effect deriving
from massless Dirac quasiparticles in graphene. Remarkably it turns out that
the zero-energy Landau levels of electrons or holes, characteristic to
graphene, remain indistinguishable in density response from the vacuum state,
although they are distinct in Hall conductance.
