# MCS 563 Spring 2014 Analytic Symbolic Computation Wednesday 9 April. Hilbert Polynomials

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1 Hilbert Polynomials For a monomial ideal, we derive the dimension counting the monomials in the complement, arriving at the notion of the Hilbert polynomial. The first half of the note is derived from [1, Chapter 9]. Following [2], cone decompositions of a ideal bound the complexity of Gröbner bases. 1 Dimension of the Variety of a Monomial Ideal The variety of a monomial ideal I is a union of coordinate planes. The dimension of this variety is the highest dimension over all planes in this union. For n variables, this dimension equals n m, where m is the minimal number of variables occurring in a generator x a of I. If I is zero dimensional, then the number of monomials not in I (visualized as the number of dots under the staircase representation of I equals the multiplicity of 0 V (I. For positive dimension I, we count the monomials not in I. For I = x a1 1 xa2 2, the complement of I consists of (a 1 + a 2 lines of monomials: { x i 1x j 2 i = 0, 1,..., a 1 1, j = 0, 1,...,a 2 1 }. For a monomial ideal I generated by more than one monomial, see for example Figure 1, we take as b 1 and b 2 the respective smallest powers of x 1 and x 2 of the generators of I and count (b 1 + b 2 lines of monomials in addition to the finite number of monomials under the staircase, counting upwards starting at (b 1, b 2. Figure 1: Black dots correspond to the generators of the monomial ideal I = x 3 1x 5 2, x 5 1x 3 2, x 6 1x 2 2. Empty circles belong to the complement of I. Observe that dividing all generators by x 3 1 x2 2 will give an ideal that has 0 as a 7-fold solution. For degree d 9, the number of monomials in the complement of I of degree d equals 5d+2 = (d+1+d+(d 1+(d+1+d+7 6 as shown by counting the monomials on the diagonal lines on the right staircase. The calculation explained in the caption of Figure 1 generalizes for any monomial ideal in two variables, leading to a linear polynomial in the degree d. This polynomial is the Hilbert polynomial. Theorem 1.1 Consider a monomial ideal I in a polynomial ring of n variables with dimv (I = d. The number of monomials not in I of degree s, for s sufficiently large, is a polynomial of degree d in s with positive coefficient of s d. To gain some intuition for why Theorem 1.1 might hold, consider ideals generated by one monomial. For example in 3 variables, consider I = x a3 3 and count all monomials not in I of degree s. These monomials lie in a 3 planes perpendicular to the third coordinate axis. Every plane holds s(s + 1/2 monomials s, so the polynomial is a 3 s(s + 1/2. For I = x a1 1 xa2 2 xa3 2, we have (a 1 + a 2 + a 3 planes of monomials not in I. For monomial ideals in 3 variables generated by more than one monomial, imagine a 3-dimensional generalization of Figure 1. As in 2 variables, we define powers b i as the smallest powers of x i in the generators of the monomial ideal. Then we count the monomials in the complement of x b1 1 xb2 2 xb3 3. For sufficiently large degrees, we add the finite number of monomials under the staircase, starting the count at (b 1, b 2, b 3. Jan Verschelde UIC, Dept of Math, Stat & CS Lecture 34, page 1

2 Proposition 1.1 For a monomial ideal I in a polynomial ring of n variables of dimension d, denote by h(s the number of monomials not in I of degree s, for s sufficiently large, then h(s = d ( a d i i=0 s d i, a i Z, a d > 0. (1 Sketch of Proof. That deg(h(s = d follows from Theorem 1.1. Furthermore h(s returns natural numbers when evaluated at natural values for s. Counting the number of monomials for d + 1 consecutive sufficiently large values of s leads to a special interpolation problem, solved by Gregory-Newton interpolation. The coefficients of the Gregory-Newton interpolating polynomial are binomial coefficients. 2 The Dimension of a Variety Given a term order >, a Gröbner basis G for an ideal I in K[x] gives a basis for the initial ideal in > (I as { LT > (g g G }. With G we have sufficiently many leading monomials for the division (or normal form algorithm to give a unique outcome. If the Hilbert polynomial for in > (I is the same as the Hilbert polynomial for I, then with a Gröbner basis we have an algorithm to compute dim(v (I. To reduce the dimension computation to counting monomials, we restrict our consideration to polynomials of bounded degree. Denote by K[x] s = { f K[x] deg(f s } and I s = { f I deg(f s } and in > (I s = { f in > (I deg(f s }. (2 With this notation, we can formally describe the If sentence as deg(h I (s = dimk[x] s /I s (3 = dimk[x] s /in > (I s (4 = deg(h in> (I (5 The equation between (3 and (4 means that the quotient rings have the same number of basis elements. Justifying this equation can happen via a Gröbner basis G > for I. An argument by contradiction assumes a different number of basis elements for the quotient rings, which then conflicts with the property of G > a Gröbner basis for I. 3 Using Macaulay 2 i1 : R = QQ[x,y]; i2 : I = ideal(x^3*y^5,x^5*y^3,x^6*y^2; i3 : h = hilbertpolynomial I o3 = 5*P 0 o3 : ProjectiveHilbertPolynomial i4 : H = hilbertpolynomial(i, Projective=>false o4 = 5 o4 : QQ[i] Jan Verschelde UIC, Dept of Math, Stat & CS Lecture 34, page 2

3 To interpret the results for our running example, we must know that Macaulay2 computes the projective Hilbert function. Let h a I (s and hp I (s be the respective affine and projective Hilbert functions. The relationship between h a I and hp I is summarized below. Theorem 3.1 h p I (s = ha I (s ha I (s 1 For our running example, we can thus explain the outcome obtained with Macaulay2 as follows: h p I (s = ha I (s ha I (s 1 = 5s + 2 (5(s = 5s + 2 (5s = 5s + 2 5s = 5. 4 The Complexity of Gröbner Bases In [2], the following theorem is proven: Theorem 4.1 Let I = f 1, f 2,...,f N be an ideal in K[x] and let d = N max i=1 deg(f i. For any term order >, the degree of polynomials required in a Gröbner basis for I with respect to > is bounded by ( d 2 2 n d. (6 As the Hilbert function counts the number of monomials in the complement of the ideal, it provides a measure for the monomials in a Gröbner basis of the ideal. 5 Cone Decompositions Let u be a subset of X = {x 1, x 2,...,x n } and f K[x] a homogeneous polynomial. The set is a cone spanned by f in u. C(f,u = { fg g K[u] } (7 The name for C(f,u is justified by a graphical representation for two variables. For example C(x 2 y 3, is just one dot at (2, 3. The cone C(x 2 y 3, {x} are all multiples for x 2 y 3, depicted as a ray extending in the left x-direction. The cone C(x 2 y 3, {x, y} is a proper cone extending both to the left and right direction. Figure 2 illustrates a collection of cones and a cone decomposition of a monomial ideal. Let {f 1, f 2,..., f r } be a set of homogeneous polynomials with {u 1,u 2,...,u r } a set of subsets of X. Then P = {(f 1,u 1, (f 2,u 2,..., (f r,u r } is a cone decomposition of R, a subring of K[x] if R = C(f 1,u 1 C(f 2,u 2 C(f r,u r, (8 i.e.: the cones C(f i,u i form a direct decomposition of R. The right of Figure 2 shows a cone decomposition of a monomial ideal. Lemma 5.1 provides an algorithm to split K[x] relative to an ideal I. Lemma 5.1 Let I be a monomial ideal, f K[x], and B is a basis for I : f. Then Jan Verschelde UIC, Dept of Math, Stat & CS Lecture 34, page 3

4 C(x 2 y, {y} C(x 3 y, {x, y} C(y 2, Figure 2: At the left we see three cones: C(y 2, is one dot, C(x 2 y 3, {y} is a ray, and the cone C(x 3 y, {x, y} has dimension 2. At the right we see the cone decomposition for I = x 4 y, xy 3, y 5 as the collection of cones {C(x 4 y, {x}, C(x 4 y 2, {x}, C(xy 3, {x, y}, C(xy 3, {x, y}, C(y 5, {y}}. 1. C(f,u I 1 B. 2. C(f,u I = B { u a a N u }. Proof B 1 I : f f I C(f, X I. 2. Assume C(f,u I =, then for any monomial u a : fu a C(f,u fu a I (9 u a I : f (10 u a B. (11 Assume B { u a a N u } =. Then for any u a : u a I : f, otherwise B would have to contain a divisor of u a of the form u b. So u a I : f fu a I. By the definition of C(f,u, every polynomial in C(f,u is of the form fu a and hence not in I. The Hilbert function of C(f,u depends only on deg(f and the size u of u: h C(f, (s = { 0 if s deg(f 1 if s = deg(f (12 and ( 0 if s < deg(f for u > 0 : h C(f,u (s = s deg(f + u 1 if s deg(f. u 1 For any cone decomposition P of R, the Hilbert function of R is h R (s = h C(f,u (s. (14 (f,u P In [2], the following expression for the Hilbert polynomial of an ideal I is derived: h I (s = ( s d + n 1 n 1 ( s d + n + n 1 + n ( s ai + i 1 i i=1 (13, (15 where the coefficients a i are the coefficients of the Hilbert polynomial of the cone decomposition. Applying the backward difference operator and other techniques leads to bounds on the coefficients of the Hilbert polynomial. Jan Verschelde UIC, Dept of Math, Stat & CS Lecture 34, page 4

5 6 Exercises 1. Use a computer algebra package to derive the Gregory-Newton interpolation formulas to interpolate (i, h i Z 2, for i = d, d + 1,...,d + n. Show that the interpolating polynomial is a linear combination of binomial coefficients. 2. Explore the command hilbertseries in Macaulay 2. References [1] D. Cox, J. Little, and D. O Shea. Ideals, Varieties and Algorithms. An Introduction to Computational Algebraic Geometry and Commutative Algebra. Undergraduate Texts in Mathematics. Springer Verlag, second edition, [2] T. Dubé. The structure of polynomial ideals and Gröbner bases. SIAM J. Comput., 19(4: , Jan Verschelde UIC, Dept of Math, Stat & CS Lecture 34, page 5

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