The Källén function, also known as triangle function, is a polynomial function in three variables, which appears in geometry and particle physics. In the latter field it is usually denoted by the symbol $\lambda$. It is named after the theoretical physicist
Gunnar Källén, who introduced it as a short-hand in his textbook Elementary Particle Physics.^{
[1]}

Definition

The function is given by a quadratic polynomial in three variables

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]}

Properties

The function is (obviously) symmetric in permutations of its arguments, as well as independent of a common sign flip of its arguments:

$\lambda (-x,-y,-z)=\lambda (x,y,z).$

If $y,z>0$ the polynomial factorizes into two factors