C*-algebras associated with some second order differential operators
  We compare two C*-algebras that have been used to study the essential spectrum. This is done by considering a simple second order elliptic differential operator acting in L^2(R^N), which is affiliated with one or both of the algebras depending on the behaviour of the coefficients. 
E Brian Davies, Vladimir Georgescu
math.SP math-ph math.MP math.OA