Lie groups are manifolds symmetric under the group action and the symmetry places uniform constraints on the global properties of the manifold. The simplest constraint resulting from the group action is on the volume growth. There are only two possibilities. In the first case the volume of a ball grows no faster than a power of its radius. Groups with this characteristic are called Lie groups of polynomial growth. Compact Lie groups fall within this class since the volume is uniformly bounded. Nilpotent Lie groups also have polynomial growth, although this is less evident, and the rate of growth is straightforwardly determined by the nilpotent structure if the group is connected and simply connected. Moreover, all Lie groups of polynomial growth are unimodular. In the second case the volume of a ball grows exponentially with its radius. All non-unimodular Lie groups have exponential growth but non-unimodularity is not essential. For example, each non-compact semisimple Lie group is unimodular but has exponential volume growth.
KeywordsHeat Equation Polynomial Growth Euclidean Group Asymptotic Evolution Subelliptic Operator
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