$L^p$ estimates for a trilinear pseudo-differential operator with flag symbols Guozhen LuLei Zhang Indiana University Mathematics Journal 2017 text pdf 10.1512/iumj.2017.66.6069 10.1512/iumj.2017.66.6069 en Indiana Univ. Math. J. 66 (2017) 877 - 900 state-of-the-art mathematics http://iumj.org/access/