https://scholar.google.com/scholar_url?url=https://www.cs.toronto.edu/~radford/csc2506/factor.ps&hl=en&sa=T&oi=gsb-gga&ct=res&cd=0&d=15860566791198058138&ei=D0zIY8-BEYaymgGdlqXIDA&scisig=AAGBfm3woRXvdFOwOZ85oU1eGHl7TfT8-g