“…For a classical presentation we refer to a treatise by Eisenhardt [9] and for a modern approach to the subject see e.g. [1,2,12,13,26,34,36,37,38] and references therein, in particular the recent books by F. Helein [23,24] and K. Kenmotsu [27]. This topic has been further explored by, among others, B. Konopelchenko [30].…”