Hilbert series and Hilbert polynomial

Definitions and main properties

Let us consider a finitely generated graded commutative algebra S over a field K, which is finitely generated by elements of positive degree. This means that

and that S 0 = K .

The Hilbert function

maps the integer n onto the dimension of the K-vector space S_n. The Hilbert series, which is called Hilbert–Poincaré series in the more general setting of graded vector spaces, is the formal series

If S is generated by h homogeneous elements of positive degrees d 1 , … , d h , then the sum of the Hilbert series is a rational fraction

where Q is a polynomial with integer coefficients.

If S is generated by elements of degree 1 then the sum of the Hilbert series may be rewritten as

where P is a polynomial with positive integer coefficients.

In this case the series expansion of this rational fraction is

where the binomial coefficient ( n + δ − 1 δ − 1 ) is ( n + δ − 1 ) ( n + δ − 2 ) ⋯ n ( δ − 1 ) ! for n > − δ and 0 otherwise.

This shows that there exists a unique polynomial H P S ( n ) with rational coefficients which is equal to H F S ( n ) for n ≥ deg ⁡ P − δ + 1 . This polynomial is the Hilbert polynomial. The least n₀ such that H P S ( n ) = H F S ( n ) for n ≥ n₀ is called the Hilbert regularity. It may be lower than deg ⁡ P − δ + 1 .

The Hilbert polynomial is a numerical polynomial, since the dimensions are integers, but the polynomial almost never has integer coefficients (Schenck 2003, pp. 41).

All these definitions may be extended to finitely generated graded modules over S, with the only difference that a factor t^m appears in the Hilbert series, where m is the minimal degree of the generators of the module, which may be negative.

The Hilbert function, the Hilbert series and the Hilbert polynomial of a filtered algebra are those of the associated graded algebra.

The Hilbert polynomial of a projective variety V in Pⁿ is defined as the Hilbert polynomial of the homogeneous coordinate ring of V.

Graded algebra and polynomial rings

Polynomial rings and their quotients by homogeneous ideals are typical graded algebras. Conversely, if S is a graded algebra generated over the field K by n homogeneous elements g₁, ..., g_n of degree 1, then the map which sends X_i onto g_i defines an homomorphism of graded rings from R n = K [ X 1 , … , X n ] onto S. Its kernel is a homogeneous ideal I and this defines an isomorphism of graded algebra between R n / I and S.

Thus, the graded algebras generated by elements of degree 1 are exactly, up to an isomorphism, the quotients of polynomial rings by homogeneous ideals. Therefore, the remainder of this article will be restricted to the quotients of polynomial rings by ideals.

Additivity

Hilbert series and Hilbert polynomial are additive relatively to exact sequences. More precisely, if

is an exact sequence of graded or filtered modules, then we have

and

This follows immediately from the same property for the dimension of vector spaces.

Quotient by a non-zero divisor

Let A be a graded algebra and f a homogeneous element of degree d in A which is not a zero divisor. Then we have

It follows immediately from the additivity on the exact sequence

where the arrow labeled f is the multiplication by f and A [ d ] is the graded module which is obtained from A by shifting the degrees by d, in order that the multiplication by f has degree 0. This implies that H S A [ d ] ( t ) = t d H S A ( t ) .

Hilbert series and Hilbert polynomial of a polynomial ring

The Hilbert series of the polynomial ring R n = K [ x 1 , … , x n ] in n indeterminates is

It follows that the Hilbert polynomial is

The proof that the Hilbert series has this simple form is obtained by applying recursively the previous formula for the quotient by a non zero divisor (here x n ) and remarking that H S K ( t ) = 1 .

Shape of the Hilbert series and dimension

A graded algebra A generated by homogeneous elements of degree 1 has Krull dimension zero if the maximal homogeneous ideal, that is the ideal generated by the homogeneous elements of degree 1, is nilpotent. This implies that the dimension of A as a K-vector space is finite and the Hilbert series of A is a polynomial P(t) such that P(1) is equal to the dimension of A as a K vector space.

If the Krull dimension of A is positive, there is a homogeneous element f of degree one which is not a zero divisor (in fact almost all elements of degree one have this property). The Krull dimension of A/(f) is the Krull dimension of A minus one.

The additivity of Hilbert series shows that H S A / ( f ) ( t ) = ( 1 − t ) H S A ( t ) . Iterating this a number of times equal to the Krull dimension of A, we get eventually an algebra of dimension 0 whose Hilbert series is a polynomial P(t). This show that the Hilbert series of A is

where the polynomial P(t) is such that P(1) ≠ 0 and d is the Krull dimension of A.

This formula for the Hilbert series implies that the degree of the Hilbert polynomial is d and that its leading coefficient is P(1)/d!.

Degree of a projective variety and Bézout's theorem

The Hilbert series allows us to compute the degree of an algebraic variety as the value at 1 of the numerator of the Hilbert series. This provides also a simple proof of Bézout's theorem. For this purpose, let us consider an projective algebraic set V defined as the set of the zeros of a homogeneous ideal I ⊂ k [ x 0 , x 1 , … , x n ] , where k is a field, and let R = k [ x 1 , … , x n ] / I be the ring of the regular functions on the algebraic set (in this section, we do not need that the algebraic set be irreducible nor that the ideal is prime).

If the dimension of V, equal to the dimension of R is d, the degree of V is the number of points of intersection, counted with multiplicity, of V with the intersection of d hyperplanes in general position. This implies that the equations of these hyperplanes, say h 1 , … , h d , are a regular sequence, and that we have the exact sequences

for k = 1 , … , d . This implies that

is a polynomial, which is equal to the numerator P ( t ) of the Hilbert series of R. After dehomogenizing by putting x 0 = 1 , Jordan-Hölder theorem for Artinian rings allows to prove that P ( 1 ) is the degree of the algebraic set V.

Similarly, if f is a homogeneous polynomial of degree δ , which is not a zero divisor in R, the exact sequence

shows that

Looking on the numerators this proves the following generalization of Bézout's theorem:

If f is a homogeneous polynomial of degree δ , which is not a zero divisor in R, then the degree of the intersection of V with the hypersurface defined by f is the product of the degree of V by δ .

The usual Bézout's theorem is easily deduced by starting from a hypersurface and intersecting it, one after the other, with n − 1 other hypersurfaces.

Computation of Hilbert series and Hilbert polynomial

The Hilbert polynomial is easily deducible from the Hilbert series. This section describes how the Hilbert series may be computed in the case of a quotient of a polynomial ring, filtered or graded by the total degree.

Thus let K a field, R = K [ x 1 , … , x n ] be a polynomial ring and I be an ideal in R. Let H be the homogeneous ideal generated by the homogeneous parts of highest degree of the elements of I. If I is homogeneous, then H=I. Finally let B be a Gröbner basis of I for a monomial ordering refining the total degree partial ordering and G the (homogeneous) ideal generated by the leading monomials of the elements of B.

The computation of the Hilbert series is based on the fact that the filtered algebra R/I and the graded algebras R/H and R/G have the same Hilbert series.

Thus the computation of the Hilbert series is reduced, through the computation of a Gröbner basis, to the same problem for an ideal generated by monomials, which is usually much easier than the computation of the Gröbner basis. The computational complexity of the whole computation depends mainly on the regularity, which is the degree of the numerator of the Hilbert series. In fact the Gröbner basis may be computed by linear algebra over the polynomials of degree bounded by the regularity.

The computation of Hilbert series and Hilbert polynomials are available in most computer algebra systems. For example in both Maple and Magma these functions are named HilbertSeries and HilbertPolynomial.

Generalization to coherent sheaves

In algebraic geometry, graded rings generated by elements of degree 1 produce projective schemes by Proj construction while finitely generated graded modules correspond to coherent sheaves. If F is a coherent sheaf over a projective scheme X, we define the Hilbert polynomial of F as a function p F ( m ) = χ ( X , F ( m ) ) , where χ is the Euler characteristic of coherent sheaf, and F ( m ) a Serre twist. Note that the Euler characteristic in this case is a well-defined number by Grothendieck's finiteness theorem.

This function is indeed a polynomial. For large m it agrees with dim H 0 ( X , F ( m ) ) by Serre's vanishing theorem. If M is a finitely generated graded module and M ~ the associated coherent sheaf two definitions of Hilbert polynomial agree.