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freeResolution(..., Strategy => Engine) -- algorithm for computing a free resolution

Synopsis

Description

This strategy chooses, depending on the ring type, an appropriate algorithm in the engine for computing free resolutions of homogeneous modules and ideals. This particular strategy is not intended for users, rather it is used internally to select an appropriate algorithm when no strategy is specified by the user.

This first example computes part of the minimal free resolution of the ground field over a hypersurface ring.

i1 : kk = ZZ/32003;
 
i2 : R = kk[a..f];
 
i3 : I = ideal(a^3-b^2*c, a^3*d - c*b^3, a*e^2-c*f^2)

             3    2      3     3      2      2
o3 = ideal (a  - b c, - b c + a d, a*e  - c*f )

o3 : Ideal of R
 
i4 : M = R^1/I

o4 = cokernel | a3-b2c -b3c+a3d ae2-cf2 |

                            1
o4 : R-module, quotient of R
 
i5 : F = freeResolution(M, Strategy => Engine)

      1      3      4      2
o5 = R  <-- R  <-- R  <-- R
                           
     0      1      2      3

o5 : Complex
 
i6 : dd^F

          1                                  3
o6 = 0 : R  <------------------------------ R  : 1
               | a3-b2c ae2-cf2 b3c-a3d |

          3                                                       4
     1 : R  <--------------------------------------------------- R  : 2
               {3} | -ae2+cf2 -b3c+a3d abe2-bcf2 b3e2-a2df2  |
               {3} | a3-b2c   0        -a3b+a3d  -a2b3+a2b2d |
               {4} | 0        a3-b2c   ae2-cf2   b2e2-a2f2   |

          4                           2
     2 : R  <----------------------- R  : 3
               {6} | a2b-a2d 0   |
               {7} | -e2     f2  |
               {7} | a2      -b2 |
               {8} | -c      a   |

o6 : ComplexMap
 
i7 : assert isWellDefined F
 
i8 : assert isQuasiIsomorphism(augmentationMap F, Concentration => (0,4))

When the input is an ideal $I$, the free resolution of $R^1/I$ is returned.

i9 : F1 = freeResolution(I, Strategy => Engine)

      1      3      4      2
o9 = R  <-- R  <-- R  <-- R
                           
     0      1      2      3

o9 : Complex
 
i10 : assert(F1 == naiveTruncation(F,0,3))
 
i11 : F2 = freeResolution(module I, Strategy => Engine)

       3      4      2
o11 = R  <-- R  <-- R
                     
      0      1      2

o11 : Complex
 
i12 : dd^F1

           1                                  3
o12 = 0 : R  <------------------------------ R  : 1
                | a3-b2c ae2-cf2 b3c-a3d |

           3                                                       4
      1 : R  <--------------------------------------------------- R  : 2
                {3} | -ae2+cf2 -b3c+a3d abe2-bcf2 b3e2-a2df2  |
                {3} | a3-b2c   0        -a3b+a3d  -a2b3+a2b2d |
                {4} | 0        a3-b2c   ae2-cf2   b2e2-a2f2   |

           4                           2
      2 : R  <----------------------- R  : 3
                {6} | a2b-a2d 0   |
                {7} | -e2     f2  |
                {7} | a2      -b2 |
                {8} | -c      a   |

o12 : ComplexMap
 
i13 : dd^F2

           3                                                       4
o13 = 0 : R  <--------------------------------------------------- R  : 1
                {3} | -ae2+cf2 b3c-a3d -ade2+cdf2 -b3e2+a2df2 |
                {4} | 0        a3-b2c  ae2-cf2    b2e2-a2f2   |
                {3} | a3-b2c   0       b3c-b2cd   a2b3-a2b2d  |

           4                           2
      1 : R  <----------------------- R  : 2
                {6} | -b3+b2d 0   |
                {7} | f2      e2  |
                {7} | -b2     -a2 |
                {8} | a       c   |

o13 : ComplexMap

The following code shows how this method chooses between strategies 1 and 2.

i14 : code (freeResolution, Module, Strategy => Engine)

o14 = -- code for strategy: freeResolution(Module,Strategy => Engine)
      /usr/local/share/Macaulay2/Complexes/
      FreeResolutions.m2:347:21-358:5: --source code:
      resolutionInEngine = (opts, M) -> (
          R := ring M;
          if isQuotientRing R or isSkewCommutative R 
               or (isWeylAlgebra R and isHomogeneous M) then (
              M.cache.ResolutionObject.Strategy = 2;
              resolutionInEngine2(opts ++ {Strategy => 2}, M)
              )
          else (
              M.cache.ResolutionObject.Strategy = 1;
              resolutionInEngine1(opts ++ {Strategy => 1}, M)
              )
          )

See also