Harmonic cochains and K-theory for $\widetilde A_2$ groups

Abstract

If $\Gamma$ is a torsion free $\widetilde A_2$ group acting on an $\widetilde A_2$ building $\Delta$, and $\fk A_{\Gamma}$ is the associated boundary $C^*$-algebra, it is proved that $K_0(\fk A_\Gamma)\otimes \bb R \cong \bb R^{2\beta_2}$, where $\beta_2=\dim_\bb R H^2(\Gamma, \bb R)$.