Open questions in cosmic-ray research at ultrahigh energies by Alves Batista, R, Biteau, J, Bustamante, M, Dolag, K, Engel, R, Fang, K, Kampert, KH, Kostunin, D, Mostafa, M, Murase, K, Oikonomou, F, Olinto, AV, Panasyuk, MI, Sigl, G, Taylor, AM, Unger, M