“…Various authors have proposed approximate methods to deal with it, as discussed in, for example, Balasubramanian (1992), Herbert and Dowsland (1996), Scheithauer and Terno (1996), Scheithauer and Sommerweiss (1998), Letchford and Amaral (2001), Maing-Kyu and Young-Gun (2001), Martins (2003), Alvarez-Valdes et al (2005a, b), Pureza and Morabito (2006) and the references therein. Upper bounds for the problem were studied in, for example, Barnes (1979); Dowsland (1985), Nelißen (1995), Letchford and Amaral (2001), Morabito and Farago (2002), Martins (2003).…”