  The Laughlin state is an ansatz for the ground state of a system of 2D
quantum particles submitted to a strong magnetic field and strong interactions.
The two effects conspire to generate strong and very specific correlations
between the particles. I present a mathematical approach to the rigidity these
correlations display in their response to perturbations. This is an important
ingredient in the theory of the fractional quantum Hall effect. The main
message is that potentials generated by impurities and residual interactions
can be taken into account by generating uncorrelated quasi-holes on top of
Laughlin's wave-function. An appendix contains a conjecture (not due to me)
that should be regarded as a major open mathematical problem of the field,
relating to the spectral gap of a certain zero-range interaction.
