It is named after the theoretical physicist Gunnar Källén, who introduced it as a short-hand in his textbook Elementary Particle Physics.
[1] The function is given by a quadratic polynomial in three variables In geometry the function describes the area
The function appears naturally in the kinematics of relativistic particles, e.g. when expressing the energy and momentum components in the center of mass frame by Mandelstam variables.
[2] The function is (obviously) symmetric in permutations of its arguments, as well as independent of a common sign flip of its arguments: If
the polynomial factorizes into four factors Its most condensed form is Interesting special cases are[2]: eqns.