# How might one formulate integration over an ordered probability simplex?

Given a probability simplex ($$x_i \geq 0$$, $$i=1,\ldots,n$$) $$\begin{equation} \sum_{i=1}^n x_i=1, \end{equation}$$ I want to attempt integrations of various ("operator-monotone"-based https://pdfs.semanticscholar.org/d393/21f142432eddd2af0d3bd07235a63aca2019.pdf) functions over that subsection for which $$\begin{equation} x_i \geq x_{i-1}. \end{equation}$$

Further, I'm particularly interested in such integrations ($$n=4$$) with the additional ("absolute separability") constraint (eq. (3) in https://arxiv.org/pdf/quant-ph/0502170.pdf) $$\begin{equation} x_1 \leq x_3+2 \sqrt{x_2 x_4}. \end{equation}$$

What are my options for setting up the integrations (coordinate transformations might be of interest)? The particular integrations I have in mind are seemingly very challenging, and I may (probably) have to resort to numerical methods.

Using the GenericCylindricalDecomposition command of Mathematica, we can express the integral of 1 over the 3-dimensional probability simplex as the sum of three integrations (for ease of presentation, I replace the subscripted variables $$x_i$$ by $$xi$$, $$i=1,2,3$$)
Integrate[1, {x3, 0, 1/4}, {x2, x3, 1/2 (1 - 2 x3)}, {x1, 1 - x2 - 2 x3, 1 - x2 - x3}] + Integrate[1, {x3, 0, 1/4}, {x2, 1/2 (1 - 2 x3), (1 - x3)/2}, {x1, x2, 1 - x2 - x3}] + Integrate[1, {x3, 1/4, 1/3}, {x2, x3, 1/2 (1 - x3)}, {x1, x2, 1 - x2 - x3}]

The results of the three integrations are $$\left\{\frac{1}{192},\frac{1}{768},\frac{1}{2304}\right\}$$, respectively, summing to $$\frac{1}{144}$$, which is one-twenty-fourth ($$\frac{1}{4!}$$)of the integration ($$\frac{1}{6}$$) over the unordered simplex.
It remains now for my investigations to incorporate the absolute separability constraint, $$\begin{equation} x_1 \leq x_3+2 \sqrt{x_2 x_4}, \end{equation}$$ into the integrations.