Envy-free divisions of a partially burnt cake

Consider n players having preferences over the connected pieces of a cake, identified with the interval [0, 1]. A classical theorem due to Stromquist ensures under mild conditions that it is possible to divide the cake into n connected pieces and assign these pieces to the players in an envy-free manner, i.e, no player strictly prefers a piece that has not been assigned to her. One of these conditions, considered as crucial, is that no player is happy with an empty piece. We prove that, even if this condition is not satisfied, it is still possible to get such a division when n is a prime number or is equal to 4. When n is at most 3, this has been previously proved by Erel Segal-Halevi, who conjectured that the result holds for any n. The main step in our proof is a new combinatorial lemma in topology, close to a conjecture by Segal-Halevi and which is reminiscent of the celebrated Sperner lemma: Instead of restricting the labels that can appear on each face of the simplex, the lemma considers labelings that enjoy a certain symmetry on the boundary.