For differential calculi on noncommutative algebras, we construct a twisted de Rham cohomology using flat connections on modules. This has properties similar, in some respects, to sheaf cohomology on topological spaces. We also discuss generalised mapping properties of these theories, and relations of these properties to corings. Using this, we give conditions for the Serre spectral sequence to hold for a noncommutative fibration. This might be better read as giving the definition of a fibration in noncommutative differential geometry. We also study the multiplicative structure of such spectral sequences. Finally we show that some noncommutative homogeneous spaces satisfy the conditions to be such a fibration, and in the process clarify the differential structure on these homogeneous spaces. We also give two explicit examples of differential fibrations: these are built on the quantum Hopf fibration with two different differential structures.
Flat connections and cohomology with twisted coefficientsThe classical Serre spectral sequence uses cohomology with a non-trivial coefficient bundle. In this section we discuss flat connections on modules in noncommutative geometry, and how this can be used to define de Rham cohomology with non-trivial coefficient modules.By a differential calculus on a noncommutative algebra A we mean a differential graded algebra (d, Ω * A) such that Ω 0 A = A. The product in Ω * A (for * ≥ 1) is denoted by the wedge ∧ (although Ω * A is not graded anticommutative in general). The density condition says that Ω n+1 A ⊂ A.dΩ n A, but we will not require this till later.The cohomology of (d, Ω * A) is denoted by H * dR (A) and referred to as a de Rham cohomology of A. Recall that a connection in a left A-module E is a map ∇ : E → Ω 1 A ⊗ A E satisfying the Leibniz rule, for all a ∈ A, e ∈ E, ∇(a.e) = da ⊗ e + a∇e.