Vandermonde polynomial


In algebra, the Vandermonde polynomial of an ordered set of n variables, named after Alexandre-Théophile Vandermonde, is the polynomial:
It is also called the Vandermonde determinant, as it is the determinant of the Vandermonde matrix.
The value depends on the order of the terms: it is an alternating polynomial, not a symmetric polynomial.

Alternating

The defining property of the Vandermonde polynomial is that it is alternating in the entries, meaning that permuting the by an odd permutation changes the sign, while permuting them by an even permutation does not change the value of the polynomial – in fact, it is the basic alternating polynomial, as will be made precise below.
It thus depends on the order, and is zero if two entries are equal – this also follows from the formula, but is also consequence of being alternating: if two variables are equal, then switching them both does not change the value and inverts the value, yielding and thus .
Conversely, the Vandermonde polynomial is a factor of every alternating polynomial: as shown above, an alternating polynomial vanishes if any two variables are equal, and thus must have as a factor for all.

Alternating polynomials

Thus, the Vandermonde polynomial generates the alternating polynomials.

Discriminant

Its square is widely called the discriminant, though some sources call the Vandermonde polynomial itself the discriminant.
The discriminant does not depend on the order of terms, as, and is thus an invariant of the unordered set of points.
If one adjoins the Vandermonde polynomial to the ring of symmetric polynomials in n variables, one obtains the quadratic extension, which is the ring of alternating polynomials.

Vandermonde polynomial of a polynomial

Given a polynomial, the Vandermonde polynomial of its roots is defined over the splitting field; for a non-monic polynomial, with leading coefficient a, one may define the Vandermonde polynomial as
to accord with the discriminant.

Generalizations

Over arbitrary rings, one instead uses a different polynomial to generate the alternating polynomials – see.

Weyl character formula

The Vandermonde polynomial can be considered a special case of the Weyl character formula, specifically the Weyl denominator formula of the special unitary group.