# How did Källén get his name attached to $x^2 + y^2 + z^2 - 2 x y -2 x z- 2 y z$?

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?

• The Wikipedia article states that the function appears in several kinematic equations, which Kallen presumably studied. – HDE 226868 Dec 24 '15 at 13:40
• @HDE226868 Thanks, I already know that (by reading the article). Would you know which equations in particular he studied, and the papers in which he studied them? – QuantumDot Dec 24 '15 at 20:26
• Nope, not "everybody" calls that function by someone's name. I've never heard a name for this function before I read it here. – KCd Dec 25 '15 at 3:35
• @KCd Thanks, that was exactly what I needed to know. – QuantumDot Dec 25 '15 at 11:44

In 1964 Källén wrote a textbook with the title "Elementary Particle Physics", which introduces the symbol with the words "The $\lambda(x,y,z)$ is a symbol which occurs frequently in this book."