Editing Lorentz factor (section)

===Rapidity===
Applying the definition of [[rapidity]] as the [[hyperbolic angle]] <math>\varphi</math>:<ref>[http://pdg.lbl.gov/2005/reviews/kinemarpp.pdf Kinematics] {{Webarchive|url=https://web.archive.org/web/20141121115205/http://pdg.lbl.gov/2005/reviews/kinemarpp.pdf |date=2014-11-21 }}, by [[John David Jackson (physicist)|J.D. Jackson]], See page 7 for definition of rapidity.</ref>
<math display="block"> \tanh \varphi = \beta</math>
also leads to {{math|γ}} (by use of [[Hyperbolic function#Useful relations|hyperbolic identities]]):
<math display="block"> \gamma = \cosh \varphi = \frac{1}{\sqrt{1 - \tanh^2 \varphi}} = \frac{1}{\sqrt{1 - \beta^2}}.</math>

Using the property of [[Lorentz transformation]], it can be shown that rapidity is additive, a useful property that velocity does not have. Thus the rapidity parameter forms a [[one-parameter group]], a foundation for physical models.