complex(ChainComplex) -- translate between data types for chain complexes

Synopsis

Function: complex
Usage:

D = complex C
Inputs:
- C, a chain complex,
Outputs:
- D, a complex,

Description

Both ChainComplex and Complex are Macaulay2 types that implement chain complexes of modules over rings. The plan is to replace ChainComplex with this new type. Before this happens, this function allows interoperability between these types.

The first example is the minimal free resolution of the twisted cubic curve.

i1 : R = ZZ/32003[a..d];

i2 : I = monomialCurveIdeal(R, {1,2,3})

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

o2 : Ideal of R

i3 : M = R^1/I

o3 = cokernel | c2-bd bc-ad b2-ac |

                            1
o3 : R-module, quotient of R

i4 : C = resolution M

      1      3      2
o4 = R  <-- R  <-- R  <-- 0
                           
     0      1      2      3

o4 : ChainComplex

i5 : D = complex C

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

o5 : Complex

i6 : D1 = freeResolution M

      1      3      2
o6 = R  <-- R  <-- R
                    
     0      1      2

o6 : Complex

i7 : assert(D == D1)

In the following example, note that a different choice of sign is chosen in the new Complexes package.

i8 : C1 = Hom(C, R^1)

             2      3      1
o8 = 0  <-- R  <-- R  <-- R
                           
     -3     -2     -1     0

o8 : ChainComplex

i9 : D1 = complex C1

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

o9 : Complex

i10 : D2 = Hom(D, R^1)

       2      3      1
o10 = R  <-- R  <-- R
                     
      -2     -1     0

o10 : Complex

i11 : D1.dd_-1

o11 = {-3} | c  -b a  |
      {-3} | -d c  -b |

              2      3
o11 : Matrix R  <-- R

i12 : D2.dd_-1

o12 = {-3} | -c b  -a |
      {-3} | d  -c b  |

              2      3
o12 : Matrix R  <-- R

i13 : assert(D1 != D2)

Caveat

This is a temporary method to allow comparisons among the data types, and will be removed once the older data structure is replaced

Ways to use this method: