Five points in 4-space with no four on a plane make a 4D simplex (or hypertetrahedron)
. A Cayley–Menger determinant uses the 10 edge lengths of
to find the hypervolume.
Five points are called flat if the hypervolume of
is zero. This is also called a 3D degenerate simplex.
Four points are called plane if the bounding volume of
is zero. This is also called a 2D degenerate tetrahedron.
Three points are called straight if the bounding area of
is zero. This is also called a 1D degenerate triangle.
This Demonstration shows 50 3D degenerate simplices with the property that all the edge lengths are powers of a particular root. Each edge is labeled with the corresponding power. The edge 0 corresponds to length
and is always the shortest length.
The plastic constant works very well (14 known solutions); it is a root of
), the smallest of the Pisot numbers. The next root that works well (10 known solutions) is a reciprocal of a root of
), the next smallest Pisot number. So far, I have found 15 different algebraic roots of degree six or less that can make a 3D power simplex.
The distances are actually powers of the square root of the given root. Index 51 has a degenerate tetrahedron, but I included it because it corresponds to the Narayana Cow constant, another of the Pisot numbers.
A degenerate 2D example uses root
, the golden ratio. The distances (0, 1, 2, 2, 1, 0) are powers of
, giving the
rectangle with diagonals of length
. Removing a point gives the Kepler triangle.
What other solutions are there where the root has degree six or less? This Demonstration is meant as a starting point for this exploration.