Some remarks on the Lp regularity of second derivatives of solutions to   non-divergence elliptic equations and the Dini condition
  In this note we prove an end-point regularity result on the Lp integrability of the second derivatives of solutions to non-divergence form uniformly elliptic equations whose second derivatives are a priori only known to be integrable. The main assumption on the elliptic operator is the Dini continuity of the coefficients. We provide a counterexample showing that the Dini condition is somehow optimal. We also give a counterexample related to the BMO regularity of second derivatives of solutions to elliptic equations. 
Luis Escauriaza, Santiago Montaner
math.AP