,j'}(0) \prod_{i=0}^{k-0} \int_0^\infty f'_{i,j}(t_i) f'_{i,j'}(t_i)\ dt_i$$ which factorizes using , as $$\begin{aligned} \beta_k &= \int_{[0,+\infty)^k} \left|\frac{\partial^k}{\partial t_0 \dots \partial t_