pub const POLY_COMPANION_CLOSED_DEG: usize = 2;Expand description
Highest degree whose companion eigenvalues have a closed form on this type.
Degree 1 is linear. Degree 2 is the characteristic polynomial of the 2×2
companion (quadratic formula). Higher-degree companions are not symmetric,
so ExactNumArray::eigen_decomp cannot be used; those degrees return
None from ExactNumPoly::roots_real.