Concept

Discriminant

A number computed from a polynomial's coefficients that vanishes exactly when the polynomial has a repeated root. It is the resultant of the polynomial and its own derivative, and its sign decides how many of the roots are real.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Also named here as symmetric function — the same set of essays touches all of them, so they are one junction rather than several.

3 roots, and the two numbers the coefficients already knew. The roots of a degree-3 polynomial, found numerically, with the point they average to. That average, and their product, are readable straight off the coefficients without finding the roots at all.

What the coefficients already know

Finding the roots of a polynomial is hard and often impossible in closed form. Reading off their sum, their product and how many of them are real is none of those things — those numbers are sitting in the coefficients, and no root-finding is required to get at them.

algebra · polynomial roots

Two coefficient arrays, one with determinant zero and one without. The Sylvester matrices of two pairs of polynomials drawn as grids of coefficients, one pair sharing a root and one not, with each determinant computed in whole numbers and checked against whether a shared root exists.

A shared root, found without finding it

Two polynomials have a root in common exactly when one determinant built from their coefficients is zero. No root is computed, nothing is approximated, and the same construction turns two equations in two unknowns into one equation in one.

algebra · polynomial roots

Named alongside it

The objects these essays reach for when they reach for this one.

Coefficient Polynomial Symmetric function Complex numbers Curve Degree Determinant Elimination Factorisation Linear map Resultant Root

All concepts