1. Department of Mathematics and Statistics, University of Western Australia, 35 Stirling Hwy, Crawley WA 6009, Australia. E-mails: serena.dipierro@uwa.edu.au, enrico.valdinoci@uwa.edu.au
2. Department of Mathematics and Computer Science, University of Florence, Viale Morgagni 67/A, 50134 Firenze, Italy. E-mail: edoardo.proiettilippi@unifi.it