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bruns -- Returns an ideal generated by three elements whose 2nd syzygy module is isomorphic to a given module

Description

This function takes a graded module M over a polynomial ring S that is a second syzygy, and returns a three-generator ideal j whose second syzygy is M, so that the resolution of S/j, from the third step, is isomorphic to the resolution of M. Alternately bruns takes a matrix whose cokernel is a second syzygy, and finds a 3-generator ideal whose second syzygy is the image of that matrix.

i1 : kk=ZZ/32003

o1 = kk

o1 : QuotientRing
 
i2 : S=kk[a..d]

o2 = S

o2 : PolynomialRing
 
i3 : i=ideal(a^2,b^2,c^2, d^2)

             2   2   2   2
o3 = ideal (a , b , c , d )

o3 : Ideal of S
 
i4 : betti (F=res i)

            0 1 2 3 4
o4 = total: 1 4 6 4 1
         0: 1 . . . .
         1: . 4 . . .
         2: . . 6 . .
         3: . . . 4 .
         4: . . . . 1

o4 : BettiTally
 
i5 : M = image F.dd_3

o5 = image {4} | c2  d2  0   0   |
           {4} | -b2 0   d2  0   |
           {4} | a2  0   0   d2  |
           {4} | 0   -b2 -c2 0   |
           {4} | 0   a2  0   -c2 |
           {4} | 0   0   a2  b2  |

                             6
o5 : S-module, submodule of S
 
i6 : f=F.dd_3

o6 = {4} | c2  d2  0   0   |
     {4} | -b2 0   d2  0   |
     {4} | a2  0   0   d2  |
     {4} | 0   -b2 -c2 0   |
     {4} | 0   a2  0   -c2 |
     {4} | 0   0   a2  b2  |

             6      4
o6 : Matrix S  <-- S
 
i7 : j=bruns M;

o7 : Ideal of S
 
i8 : betti res j -- the ideal has 3 generators

            0 1 2 3 4
o8 = total: 1 3 5 4 1
         0: 1 . . . .
         1: . . . . .
         2: . . . . .
         3: . 3 . . .
         4: . . . . .
         5: . . . . .
         6: . . 5 . .
         7: . . . 4 .
         8: . . . . 1

o8 : BettiTally

Here is a more complicated example, also involving a complete intersection. You can see that columns three and four in the two Betti diagrams are the same.

i9 : kk=ZZ/32003

o9 = kk

o9 : QuotientRing
 
i10 : S=kk[a..d]

o10 = S

o10 : PolynomialRing
 
i11 : i=ideal(a^2,b^3,c^4, d^5)

              2   3   4   5
o11 = ideal (a , b , c , d )

o11 : Ideal of S
 
i12 : betti (F=res i)

             0 1 2 3 4
o12 = total: 1 4 6 4 1
          0: 1 . . . .
          1: . 1 . . .
          2: . 1 . . .
          3: . 1 1 . .
          4: . 1 1 . .
          5: . . 2 . .
          6: . . 1 1 .
          7: . . 1 1 .
          8: . . . 1 .
          9: . . . 1 .
         10: . . . . 1

o12 : BettiTally
 
i13 : M = image F.dd_3

o13 = image {5} | c4  d5  0   0   |
            {6} | -b3 0   d5  0   |
            {7} | a2  0   0   d5  |
            {7} | 0   -b3 -c4 0   |
            {8} | 0   a2  0   -c4 |
            {9} | 0   0   a2  b3  |

                              6
o13 : S-module, submodule of S
 
i14 : f=F.dd_3

o14 = {5} | c4  d5  0   0   |
      {6} | -b3 0   d5  0   |
      {7} | a2  0   0   d5  |
      {7} | 0   -b3 -c4 0   |
      {8} | 0   a2  0   -c4 |
      {9} | 0   0   a2  b3  |

              6      4
o14 : Matrix S  <-- S
 
i15 : j1=bruns f;

o15 : Ideal of S
 
i16 : betti res j1

             0 1 2 3 4
o16 = total: 1 3 5 4 1
          0: 1 . . . .
          1: . . . . .
          2: . . . . .
          3: . . . . .
          4: . . . . .
          5: . . . . .
          6: . . . . .
          7: . . . . .
          8: . 1 . . .
          9: . 2 . . .
         10: . . . . .
         11: . . . . .
         12: . . . . .
         13: . . . . .
         14: . . . . .
         15: . . 1 . .
         16: . . 1 . .
         17: . . 2 . .
         18: . . 1 1 .
         19: . . . 1 .
         20: . . . 1 .
         21: . . . 1 .
         22: . . . . 1

o16 : BettiTally
 
i17 : j=bruns M;

o17 : Ideal of S
 
i18 : betti res j

             0 1 2 3 4
o18 = total: 1 3 5 4 1
          0: 1 . . . .
          1: . . . . .
          2: . . . . .
          3: . . . . .
          4: . . . . .
          5: . . . . .
          6: . . . . .
          7: . . . . .
          8: . 1 . . .
          9: . 2 . . .
         10: . . . . .
         11: . . . . .
         12: . . . . .
         13: . . . . .
         14: . . . . .
         15: . . 1 . .
         16: . . 1 . .
         17: . . 2 . .
         18: . . 1 1 .
         19: . . . 1 .
         20: . . . 1 .
         21: . . . 1 .
         22: . . . . 1

o18 : BettiTally

In the next example, we perform the "Brunsification" of a rational curve.

i19 : kk=ZZ/32003

o19 = kk

o19 : QuotientRing
 
i20 : S=kk[a..e]

o20 = S

o20 : PolynomialRing
 
i21 : i=monomialCurveIdeal(S, {1,3,4,5})

              2                    2                            2    3    2
o21 = ideal (d  - c*e, b*d - a*e, c  - b*e, b*c - a*d, a*c*d - b e, b  - a c)

o21 : Ideal of S
 
i22 : betti (F=res i)

             0 1 2 3 4
o22 = total: 1 5 8 5 1
          0: 1 . . . .
          1: . 4 2 . .
          2: . 1 6 5 1

o22 : BettiTally
 
i23 : time j=bruns F.dd_3;
 -- used 0.195181s (cpu); 0.195621s (thread); 0s (gc)

o23 : Ideal of S
 
i24 : betti res j

             0 1 2 3 4
o24 = total: 1 3 6 5 1
          0: 1 . . . .
          1: . . . . .
          2: . . . . .
          3: . . . . .
          4: . 3 . . .
          5: . . . . .
          6: . . . . .
          7: . . 2 . .
          8: . . 4 5 1

o24 : BettiTally

See also

Ways to use bruns:

  • bruns(Matrix)
  • bruns(Module)

For the programmer

The object bruns is a method function.

The source of this document is in Bruns.m2:288:0.