1. Department of Mathematics, The University of Tulsa, 800 South Tucker Drive, Tulsa, Oklahoma 74104, USA. E-mail: matteo-dallariva@utulsa.edu
2. Department of Mathematics, Aberystwyth University, Ceredigion SY23 3BZ, Wales, UK
3. Dipartimento di Matematica ‘Tullio Levi-Civita’, Università degli Studi di Padova, Via Trieste 63, Padova 35121, Italy. E-mails: musolino@math.unipd.it, rpukhtaievych@gmail.com