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Truncations :: truncate

truncate -- truncation of the graded ring, ideal or module at a specified degree or set of degrees

Synopsis

Description

The truncation to degree d in the singly graded case of a module (or ring or ideal) is generated by all homogeneous elements of degree at least d in M. The resulting truncation is minimally generated (assuming that M is graded).

i1 : R = ZZ/101[a..c];
i2 : truncate(2, R)

             2             2        2
o2 = ideal (c , b*c, a*c, b , a*b, a )

o2 : Ideal of R
i3 : truncate(2,R^1)

o3 = image | c2 bc ac b2 ab a2 |

                             1
o3 : R-module, submodule of R
i4 : truncate(2,R^1 ++ R^{-3})

o4 = image {0} | c2 bc ac b2 ab a2 0 |
           {3} | 0  0  0  0  0  0  1 |

                             2
o4 : R-module, submodule of R
i5 : truncate(2, ideal(a,b,c^3)/ideal(a^2,b^2,c^4))

o5 = subquotient (| bc ac ab c3 |, | b2 a2 c4 |)

                               1
o5 : R-module, subquotient of R
i6 : truncate(2,ideal(a,b*c,c^7))

                            2   7
o6 = ideal (b*c, a*c, a*b, a , c )

o6 : Ideal of R
i7 : M = coker matrix"a,b,c;c,b,a"

o7 = cokernel | a b c |
              | c b a |

                            2
o7 : R-module, quotient of R
i8 : truncate(2, M)

o8 = subquotient (| c2 bc b2 0  |, | a b c |)
                  | 0  0  0  c2 |  | c b a |

                               2
o8 : R-module, subquotient of R
i9 : M/(truncate(2,M))

o9 = cokernel | c2 bc b2 0  a b c |
              | 0  0  0  c2 c b a |

                            2
o9 : R-module, quotient of R
i10 : for i from 0 to 5 list hilbertFunction(i,oo)

o10 = {2, 3, 0, 0, 0, 0}

o10 : List

The base may be ZZ, or another polynomial ring. Over ZZ, the generators may not be minimal, but they do generate.

i11 : A = ZZ[x,y,z];
i12 : truncate(2,ideal(3*x,5*y,15))

                2                2         2
o12 = ideal (15z , 5y*z, 3x*z, 5y , x*y, 3x )

o12 : Ideal of A
i13 : trim oo

                2                2         2
o13 = ideal (15z , 5y*z, 3x*z, 5y , x*y, 3x )

o13 : Ideal of A
i14 : truncate(2,comodule ideal(3*x,5*y,15))

o14 = subquotient (| z2 yz xz y2 x2 |, | 15 5y 3x xy |)

                                1
o14 : A-module, subquotient of A

If i is a multi-degree, then the result is the submodule generated by all elements of degree (component-wise) greater than or equal to i.

The following example finds the 11 generators needed to obtain all graded elements whose degrees are component-wise at least {7,24}.

i15 : S = ZZ/101[x,y,z,Degrees=>{{1,3},{1,4},{1,0}}];
i16 : trunc = truncate({7,24}, S^1 ++ S^{{-8,-20}})

o16 = image {0, 0}  | y6z y7 xy6 x2y5 x3y4 x4y3 x6y2 x7y x8 0 0  |
            {8, 20} | 0   0  0   0    0    0    0    0   0  y x2 |

                              2
o16 : S-module, submodule of S
i17 : degrees trunc

o17 = {{7, 24}, {7, 28}, {7, 27}, {7, 26}, {7, 25}, {7, 24}, {8, 26}, {8,
      -----------------------------------------------------------------------
      25}, {8, 24}, {9, 24}, {10, 26}}

o17 : List

If i is a list of multi-degrees, then the result is the submodule generated by all elements of degree (component-wise) greater than or equal to at least one degree in i.

The following example finds the generators needed to obtain all graded elements whose degrees which are component-wise at least {3,0} or at least {0,1}. The resulting module is also minimally generated.

i18 : S = ZZ/101[x,y,z,Degrees=>{{1,3},{1,4},{1,0}}];
i19 : trunc = truncate({{3,0},{0,1}}, S^1 ++ S^{{-8,-20}})

o19 = image {0, 0}  | y x z3 0 |
            {8, 20} | 0 0 0  1 |

                              2
o19 : S-module, submodule of S
i20 : degrees trunc

o20 = {{1, 4}, {1, 3}, {3, 0}, {8, 20}}

o20 : List

The coefficient ring may also be a polynomial ring. In this example, the coefficient variables also have degree one. The given generators will generate the truncation over the coefficient ring.

i21 : B = R[x,y,z, Join=>false]

o21 = B

o21 : PolynomialRing
i22 : degree x

o22 = {1}

o22 : List
i23 : degree B_3

o23 = {1}

o23 : List
i24 : truncate(2, B^1)

o24 = image | c2 bc ac b2 ab a2 cz bz az cy by ay cx bx ax z2 yz xz y2 xy x2 |

                              1
o24 : B-module, submodule of B
i25 : truncate(4, ideal(b^2*y,x^3))

              2      3      2    2      2 2   2        3     3     3   3  
o25 = ideal (b c*y, b y, a*b y, b y*z, b y , b x*y, c*x , b*x , a*x , x z,
      -----------------------------------------------------------------------
       3    4
      x y, x )

o25 : Ideal of B

If the coefficient variables have degree 0:

i26 : A1 = ZZ/101[a,b,c,Degrees=>{3:{}}]

o26 = A1

o26 : PolynomialRing
i27 : degree a

o27 = {}

o27 : List
i28 : B1 = A1[x,y]

o28 = B1

o28 : PolynomialRing
i29 : degrees B1

o29 = {{1}, {1}}

o29 : List
i30 : truncate(2,B1^1)

o30 = image | y2 xy x2 |

                                1
o30 : B1-module, submodule of B1
i31 : truncate(2, ideal(a^3*x, b*y^2))

                2   3      3 2
o31 = ideal (b*y , a x*y, a x )

o31 : Ideal of B1

Caveat

The behavior of this function has changed as of Macaulay2 version 1.13. This is a (potentially) breaking change. Before, it used a less useful notion of truncation, involving the heft vector, and was often not what one wanted in the multi-graded case. Additionally, in the tower ring case, when the coefficient ring had variables of nonzero degree, sometimes incorrect answers resulted.

Also, the function expects a graded module, ring, or ideal, but this is not checked, and some answer is returned.

See also

Ways to use truncate :