On the second iterate for active scalar equations
  We consider an iterative resolution scheme for a broad class of active scalar equations with a fractional power \gamma of the Laplacian and focus our attention on the second iterate. The main objective of our work is to analyze boundedness properties of the resulting bilinear operator, especially in the super-critical regime. Our results are two-fold: we prove continuity of the bilinear operator in BMO^{1-2\gamma} - a fractional analogue of the Koch-Tataru space; for equations with an even symbol we show that the B^{-\gamma}_{\infty,q} -regularity, where q > 2, is in a sense a minimal necessary requirement on the solution. 
Susan Friedlander and Walter Rusin
math.AP