Toric ideals#
A toric ideal (associated to an integer matrix \(A\)) is an ideal of the form
In other words, it is an ideal generated by irreducible “binomials”, that is, differences of monomials without a common factor. Since the Buchberger algorithm preserves this property, any Groebner basis is then also generated by binomials.
EXAMPLES:
sage: A = matrix([[1,1,1], [0,1,2]])
sage: IA = ToricIdeal(A)
sage: IA.ker()
Free module of degree 3 and rank 1 over Integer Ring
User basis matrix:
[-1 2 -1]
sage: IA
Ideal (-z1^2 + z0*z2) of Multivariate Polynomial
Ring in z0, z1, z2 over Rational Field
Here, the “naive” ideal generated by \(z_0 z_2 - z_1^2\) does already equal the toric ideal. But that is not true in general! For example, this toric ideal ([Stu1997], Example 1.2) is the twisted cubic and cannot be generated by \(2=\dim \ker(A)\) polynomials:
sage: A = matrix([[3,2,1,0], [0,1,2,3]])
sage: IA = ToricIdeal(A)
sage: IA.ker()
Free module of degree 4 and rank 2 over Integer Ring
User basis matrix:
[-1 1 1 -1]
[-1 2 -1 0]
sage: IA
Ideal (-z1*z2 + z0*z3, -z1^2 + z0*z2, z2^2 - z1*z3) of
Multivariate Polynomial Ring in z0, z1, z2, z3 over Rational Field
The following family of toric ideals is from Example 4.4 of [Stu1997]. One can show that \(I_d\) is generated by one quadric and \(d\) binomials of degree \(d\):
sage: def I(d):
....: return ToricIdeal(matrix([[1,1,1,1,1], [0,1,1,0,0], [0,0,1,1,d]]))
sage: I(2)
Ideal (-z3^2 + z0*z4,
z0*z2 - z1*z3,
z2*z3 - z1*z4) of
Multivariate Polynomial Ring in z0, z1, z2, z3, z4 over Rational Field
sage: I(3)
Ideal (-z3^3 + z0^2*z4,
z0*z2 - z1*z3,
z2*z3^2 - z0*z1*z4,
z2^2*z3 - z1^2*z4) of
Multivariate Polynomial Ring in z0, z1, z2, z3, z4 over Rational Field
sage: I(4)
Ideal (-z3^4 + z0^3*z4,
z0*z2 - z1*z3,
z2*z3^3 - z0^2*z1*z4,
z2^2*z3^2 - z0*z1^2*z4,
z2^3*z3 - z1^3*z4) of
Multivariate Polynomial Ring in z0, z1, z2, z3, z4 over Rational Field
Finally, the example in [SH1995b]
sage: A = matrix(ZZ, [ [15, 4, 14, 19, 2, 1, 10, 17],
....: [18, 11, 13, 5, 16, 16, 8, 19],
....: [11, 7, 8, 19, 15, 18, 14, 6],
....: [17, 10, 13, 17, 16, 14, 15, 18] ])
sage: IA = ToricIdeal(A) # long time
sage: IA.ngens() # long time
213
AUTHORS:
Volker Braun (2011-01-03): Initial version
- class sage.schemes.toric.ideal.ToricIdeal(A, names='z', base_ring=Rational Field, polynomial_ring=None, algorithm='HostenSturmfels')#
Bases:
MPolynomialIdeal
This class represents a toric ideal defined by an integral matrix.
INPUT:
A
– integer matrix. The defining matrix of the toric ideal.names
– string (optional). Names for the variables. By default, this is'z'
and the variables will be namedz0
,z1
, …base_ring
– a ring (optional). Default: \(\QQ\). The base ring of the ideal. A toric ideal uses only coefficients \(\pm 1\).polynomial_ring
– a polynomial ring (optional). The polynomial ring to construct the ideal in.You may specify the ambient polynomial ring via the
polynomial_ring
parameter or via thenames
andbase_ring
parameter. AValueError
is raised if you specify both.algorithm
– string (optional). The algorithm to use. For now, must be'HostenSturmfels'
which is the algorithm proposed by Hosten and Sturmfels in [SH1995b].
EXAMPLES:
sage: A = matrix([[1,1,1], [0,1,2]]) sage: ToricIdeal(A) Ideal (-z1^2 + z0*z2) of Multivariate Polynomial Ring in z0, z1, z2 over Rational Field
First way of specifying the polynomial ring:
sage: ToricIdeal(A, names='x,y,z', base_ring=ZZ) Ideal (-y^2 + x*z) of Multivariate Polynomial Ring in x, y, z over Integer Ring
Second way of specifying the polynomial ring:
sage: R.<x,y,z> = ZZ[] sage: ToricIdeal(A, polynomial_ring=R) Ideal (-y^2 + x*z) of Multivariate Polynomial Ring in x, y, z over Integer Ring
It is an error to specify both:
sage: ToricIdeal(A, names='x,y,z', polynomial_ring=R) Traceback (most recent call last): ... ValueError: you must not specify both variable names and a polynomial ring
- A()#
Return the defining matrix.
OUTPUT: An integer matrix.
EXAMPLES:
sage: A = matrix([[1,1,1], [0,1,2]]) sage: IA = ToricIdeal(A) sage: IA.A() [1 1 1] [0 1 2]
- ker()#
Return the kernel of the defining matrix.
OUTPUT: The kernel of
self.A()
.EXAMPLES:
sage: A = matrix([[1,1,1], [0,1,2]]) sage: IA = ToricIdeal(A) sage: IA.ker() Free module of degree 3 and rank 1 over Integer Ring User basis matrix: [-1 2 -1]