tetrahedra

[nschloe]

Singularity-free evaluation of collapsed-coordinate orthogonal polynomials, Robert C. Kirby, Texas Tech University
https://cpb-us-w2.wpmucdn.com/sites.baylor.edu/dist/6/32/files/2015/01/dubrec-1dzeylu.pdf

[nschloe]

See also chapter 2.5.2 (Classical orthogonal polynomials on the simplex) in
Orthogonal polynomials of several variables, Yuan Xu

This actually generalizes the 2D case in orthopy.