1. Department of Mathematics and Statistics, Indian Institute of Technology – Kanpur, Kalyanpur, Kanpur – 208016, India.
2. Institutt for matematiske fag, NTNU, 7491 Trondheim, Norway.
3. Instytut Matematyczny, Uniwersytet Wrocławski, 50-384 Wrocław, Poland, and Prirodoslovno–matematički fakultet, Sveučilište u Zagrebu, 10000 Zagreb, Croatia.