@techreport {TR-1980-023,
   type = {Technical Report},
   author = {Arthur Farley, Andrzej Proskurowski},
   title = {The Complexity of Determining the Line Index of a Signed Graph},
   institution = {University of Oregon, Computer and Information Sciences Department},
   number = TR-1980-023,
   year = 1980,
   note = {Available at \url{https://www.cs.uoregon.edu/Reports/TR-1980-023.pdf}},
}