Quantitative uniqueness for the power of Laplacian with singular   coefficients
  In this paper we study the local behavior of a solution to the $l$th power of Laplacian with singular coefficients in lower order terms. We obtain a bound on the vanishing order of the nontrivial solution. Our proofs use Carleman estimates with carefully chosen weights. We will derive appropriate three-sphere inequalities and apply them to obtain doubling inequalities and the maximal vanishing order. 
Ching-Lung Lin, Sei Nagayasu, and Jenn-Nan Wang