Chern-Weil theory for singular Haefliger foliations

We give a Chern-Weil map for the Gel'fand-Fuks characteristic classes of densely regular foliations, those foliations defined by smooth Haefliger structures with dense regular set. Our characteristic map constructs, out of singular geometric structures adapted to singularities, explicit forms representing characteristic classes in de Rham cohomology. The forms are functorial under foliation morphisms. We prove that the theory applies, up to homotopy, to general smooth Haefliger structures: subject only to obvious necessary dimension constraints, every smooth Haefliger structure is homotopic to a densely regular foliation. As an application, we provide a generalisation to the singular setting of the classical construction of forms representing the Godbillon-Vey invariant.