Hardy and BMO spaces associated to divergence form elliptic operators
  Consider the second order divergence form elliptic operator $L$ with complex bounded coefficients. In general, the operators related to it (such as Riesz transform or square function) lie beyond the scope of the Calder\'{o}n-Zygmund theory. They need not be bounded in the classical Hardy, BMO and even some $L^p$ spaces.   In this work we generalize the classical approach and develop a theory of Hardy and BMO spaces associated to $L$, which includes, in particular, molecular decomposition, maximal function characterization, duality of Hardy and BMO spaces, John-Nirenberg inequality, and allows to handle aforementioned operators. 
Steve Hofmann, Svitlana Mayboroda