Eisenstein cohomology for Bianchi 3-manifolds

A joint conference and number theory lunch seminar talk.
Contact: Pieter Moree (moree@mpim-bonn.mpg.de).

I will review work of Harder and others on the Eisenstein cohomology for subgroups \(\Gamma \le \operatorname{SL}(2)\) with entries in the ring of integers of an imaginary quadratic field. For proving congruences of Eisenstein and cuspidal cohomology classes one needs to analyze the restriction of integral cohomology to the boundary of the Borel-Serre compactification of the 3-manifold \(\mathbb{H}^3/ \Gamma\). I will report on ongoing joint work with Adel Betina (Vienna) on applying this to prove congruences modulo inert primes for classical CM modular forms.