@article{JEMS_2017_19_5_a2,
     author = {Tadeusz Kulczycki},
     title = {On concavity of solutions of the {Dirichlet} problem for the equation $(-\Delta)^{1/2} \varphi = 1$ in convex planar regions},
     journal = {Journal of the European Mathematical Society},
     pages = {1361--1420},
     publisher = {mathdoc},
     volume = {19},
     number = {5},
     year = {2017},
     doi = {10.4171/jems/695},
     url = {http://geodesic.mathdoc.fr/articles/10.4171/jems/695/}
}