On the Dimension of Cohomology of Bianchi Groups
  Using Lefschetz numbers of certain involutions, we provide lower bounds for the cuspidal cohomology of principal congruence subgroups of Bianchi groups. The asymptotic lower bounds that follow from our results complement recent results of Calegari-Emerton, Marshall and Finis-Grunewald-Tirao. Moreover, we discuss the relationship between these involutions and the base change classes in the cohomology. 
Mehmet Haluk Sengun, Seyfi Turkelli
math.NT math.GN math.GR