Hi,

I am reading R. Hagedorn’s Relativistic Kinematics (1964) book in which he derived the 2-body phase-space R2(a;b,c) expression (Equation 7-57 in Chapter 7). Looking at the TGenPhaseSpace class

https://root.cern.ch/doc/master/TGenPhaseSpace_8cxx_source.html#l00034

the PDK function seems to evaluate this R2 phase-space factor if I am not mistaken. However, in the denominator there is only a factor “2*a”

```
Double_t x = (a-b-c)*(a+b+c)*(a-b+c)*(a+b-c);
x = TMath::Sqrt(x)/(2*a);
return x;
```

while in the Hagedorn book the denominator reads “2*a*a”. I am wondering if I misinterpret the code or perhaps the missing factor is included elsewhere in the class.

Sincerely,

BR