<ref>{{cite book |author1=Medio, Alfredo |author2=Lines, Marji |title=Nonlinear Dynamics: A Primer |publisher=Cambridge University Press |year=2001 |isbn=978-0-521-55874-7 |page=165 }}</ref> | <ref>{{cite book |author1=Medio, Alfredo |author2=Lines, Marji |title=Nonlinear Dynamics: A Primer |publisher=Cambridge University Press |year=2001 |isbn=978-0-521-55874-7 |page=165 }}</ref> |