On page 92 of [these notes](https://web.math.princeton.edu/%7Eseri/homepage/courses/Analysis2011.pdf), there is a discussion on how to find the fundamental solution to the D'Alembertian operator. It is firstly proposed that $c\_n(t^2 - |x|^2 )^{-(n-1)/2}$ may be a good candidate. Then the author says that we can use the homogeneous family of distributions $j\_a$ to get a homogeneous distribution that agrees with the above function in $t>|x|$.


However, I am unable to find the definition of $j\_a.$


Could anyone **give me a reference/clue** on what result we are using here?



Classification of homogeneous distributions
In sergiu's notes that you referred to, $j\_a$ is defined in Definition 3.2 on Page 65. See equation (134).


