For some reason, everybody refers to the function $$\lambda(x,y,z) = x^2 + y^2 + z^2 - 2xy -2xz-2yz$$ as "Källén's triangle function." (see for example https://en.wikipedia.org/wiki/Gunnar_Källén). I understand why its a triangle function (because it's proportional the area of a triangle of sides $x$ $y$ and $z$).
But how did Källén get his name attached to it? Is there a paper of his where he used that combination a lot?