@article{JEMS_2015_17_9_a2,
     author = {Matthew J. Gursky and Andrea Malchiodi},
     title = {A strong maximum principle for the {Paneitz} operator and a non-local flow for the $Q$-curvature},
     journal = {Journal of the European Mathematical Society},
     pages = {2137--2173},
     publisher = {mathdoc},
     volume = {17},
     number = {9},
     year = {2015},
     doi = {10.4171/jems/553},
     url = {http://geodesic.mathdoc.fr/articles/10.4171/jems/553/}
}