Nonnegative quadratic Bézier triangular patches (Q1315813)

A quadratic triangular Bézier patch is positive, i.e., the surface represented by the patch will not cross the plane of the triangle at interior points of the triangle, if a certain 3-by-3 symmetric matrix is copositive [cf. \textit{K. P. Hadeler}, Linear Algebra Appl. 49, 79-89 (1983; Zbl 0506.15016)]. In this particular lowdimensional case the authors give a complete algebraic characterization without using matrix theory.