Title: Existence of Mexican-hat dispersion and symmetry group of a layer

Abstract: Increased interest in physics of graphene and other two-dimensional materials boosted investigations of band structure near nodal points and lines. In contrast, group theoretical explanation of simple bands (that do not touch other bands), is sporadically present in the literature. This paper presents electronic dispersions up to forth order in momentum, near Brillouin zone (BZ) high symmetry points of all eighty layer groups. The method applies to non magnetic materials both with or without spin-orbit coupling. Particular attention is devoted to Mexican-hat dispersion, showing that it can appear only at BZ center of hexagonal layer groups. Presented symmetry adapted Taylor expansion of bands can be used to fit ab-initio or experimental band structures, or for analytical calculation of crystal properties. The results presented here might serve also as a guiding tool for design of new two-dimensional materials.