Open questions in cosmic-ray research at ultrahigh energies par 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