“…There are many possible ways to study the Hele-Shaw equation: to mention a few approaches we quote various PDE methods based on L 2 -energy estimates (see the works of Chen [15], Córdoba, Córdoba and Gancedo [24], Knüpfer and Masmoudi [36], Günther and Prokert [33], Cheng, Granero-Belinchón and Shkoller [16]), there are also methods based on functional analysis tools and maximal estimates (see Escher and Simonett [30], the results reviewed in the book by Prüss and Simonett [42] and Matioc [38,39]) or methods using harmonic analysis tools and contour integrals (see the numerous results reviewed in the survey papers by Gancedo [31] or Granero-Belinchón and Lazar [32]). For the related Muskat equation (a two-phase Hele-Shaw problem), maximum principles have played a key role to study the Cauchy problem, see [12,18,10,26] following the pioneering work of Constantin, Córdoba, Gancedo, Rodríguez-Piazza and Strain [17]. Such maximum principles have been obtained for general viscosity solutions of the Hele-Shaw equation by Kim [35], see also the recent work of Chang-Lara, Guillen and Schwab [14].…”