Examples of arithmetic groups include therefore the groups GLn(Z). The idea of arithmetic group is closely related to that of lattice in a Lie group. Lattices in that sense tend to be arithmetic, except in well-defined circumstances. The exact relationship of the two concepts was established by the work of Margulis on superrigidity. The general theory of arithmetic groups was developed by Armand Borel and Harish-Chandra; the description of their fundamental domains was in classical terms the reduction theory of algebraic forms.