Parabolic equations with divergence-free drift in space $L_{t}^{l}L_{x}^{q} Zhongmin Qian Indiana University Mathematics Journal 2019 text pdf 10.1512/iumj.2019.68.7685 10.1512/iumj.2019.68.7685 en Indiana Univ. Math. J. 68 (2019) 761 - 797 state-of-the-art mathematics http://iumj.org/access/