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# Counting points in discrete subgroups (Jeff Vaaler, UT Austin)

## November 2, 2021 @ 12:30 pm - 1:20 pm

We consider the problem of comparing the number of discrete points that belong to a set with the measure (or volume) of the set, under circumstances where we expect these two numbers to be approximately equal. We start with a locally compact, abelian, topological group G. We assume that G has a countably infinite, torsion free, discrete subgroup H. But to make the talk easier to follow we will mostly consider the case G = R^N and H = Z^N. If E ⊆ R^N is a subset there are many situations where one expects that the (finite, positive) number Vol_N (E) is approximately equal to the cardinality |E ∩ Z^N |. We will sketch the proof of a general result that bounds the difference between these quantities. If k is an algebraic number field and k_A is the ring of adeles associated to k, this general result is useful when G = k_A^N and H = k^N .