How to make filtered complexes from chain complex maps

We describe the most primitive way to create filtered complexes.

Let $C$ be a chain complex and consider a list of chain complex maps $\{\phi_n, \phi_{n - 1}, \dots, \phi_0 \}$ with properties that $C$ is the target of $\phi_i$, for $0 \leq i \leq n$, and the image of $\phi_{i-1}$ is a subchain complex of the image of $\phi_i$, for $1 \leq i \leq n$. Given this input data we produce an ascending filtered chain complex $FC$ with the properties that $F_k C = C$ for $k \geq n + 1$ and $F_k C = image \phi_k$, for $k = 0, \dots, n$.

We now illustrate how this is done in two easy examples. We first make three chain complexes $C$, $D$, and $E$, two chain complex maps, $d : D \rightarrow C$ and $e : E \rightarrow C$, and then compute the resulting filtration of $C$.

Let's make our chain complexes $C$, $D$, and $E$.

i1 : R = QQ[x,y,z,w] ;

i2 : c2 = matrix(R,{{1},{0}}) ;

             2      1
o2 : Matrix R  <-- R

i3 : c1 = matrix(R,{{0,1}}) ;

             1      2
o3 : Matrix R  <-- R

i4 : C = chainComplex({c1,c2})

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

o4 : ChainComplex

i5 : D_2 = image matrix(R,{{1}});

i6 : D_1 = image matrix(R,{{1,0},{0,0}});

i7 : D_0 = image matrix(R,{{1}});

i8 : D = chainComplex({inducedMap(D_0,D_1,C.dd_1),inducedMap(D_1,D_2,C.dd_2)})

o8 = image | 1 | <-- image | 1 0 | <-- image | 1 |
                           | 0 0 |      
     0                                 2
                     1

o8 : ChainComplex

i9 : E_2 = image matrix(R,{{0}});

i10 : E_1 = image matrix(R,{{1,0},{0,0}});

i11 : E_0 = image matrix(R,{{1}});

i12 : E = chainComplex({inducedMap(E_0,E_1,C.dd_1),inducedMap(E_1,E_2,C.dd_2)})

o12 = image | 1 | <-- image | 1 0 | <-- image 0
                            | 0 0 |      
      0                                 2
                      1

o12 : ChainComplex

We now make our chain complex maps.

i13 : d = chainComplexMap(C,D,apply(spots C, i-> inducedMap(C_i,D_i,id_C _i)))

           1
o13 = 0 : R  <--------- image | 1 | : 0
                | 1 |

           2
      1 : R  <----------- image | 1 0 | : 1
                | 1 0 |         | 0 0 |
                | 0 0 |

           1
      2 : R  <--------- image | 1 | : 2
                | 1 |

o13 : ChainComplexMap

i14 : e = chainComplexMap(C,E,apply(spots C, i->inducedMap(C_i,E_i, id_C _i)))

           1
o14 = 0 : R  <--------- image | 1 | : 0
                | 1 |

           2
      1 : R  <----------- image | 1 0 | : 1
                | 1 0 |         | 0 0 |
                | 0 0 |

           1
      2 : R  <----- image 0 : 2
                0

o14 : ChainComplexMap

We can check that these are indeed chain complex maps:

i15 : isChainComplexMap d

o15 = true

i16 : isChainComplexMap e

o16 = true

Now, given the list of chain complex maps $\{d, e\}$, we obtain a filtration of $C$ by:

i17 : K = filteredComplex({d,e})

o17 = -1 : image 0 <-- image 0 <-- image 0
                                    
           0           1           2

      0 : image | 1 | <-- image | 1 0 | <-- image 0
                                | 0 0 |      
          0                                 2
                          1

      1 : image | 1 | <-- image | 1 0 | <-- image | 1 |
                                | 0 0 |      
          0                                 2
                          1

           1      2      1
      2 : R  <-- R  <-- R
                         
          0      1      2

o17 : FilteredComplex

If we want to specify a minimum filtration degree we can use the Shift option.

i18 : L = filteredComplex({d,e},Shift =>1)

o18 = -2 : image 0 <-- image 0 <-- image 0
                                    
           0           1           2

      -1 : image | 1 | <-- image | 1 0 | <-- image 0
                                 | 0 0 |      
           0                                 2
                           1

      0 : image | 1 | <-- image | 1 0 | <-- image | 1 |
                                | 0 0 |      
          0                                 2
                          1

           1      2      1
      1 : R  <-- R  <-- R
                         
          0      1      2

o18 : FilteredComplex

i19 : M = filteredComplex({d,e},Shift =>-1)

o19 = 0 : image 0 <-- image 0 <-- image 0
                                   
          0           1           2

      1 : image | 1 | <-- image | 1 0 | <-- image 0
                                | 0 0 |      
          0                                 2
                          1

      2 : image | 1 | <-- image | 1 0 | <-- image | 1 |
                                | 0 0 |      
          0                                 2
                          1

           1      2      1
      3 : R  <-- R  <-- R
                         
          0      1      2

o19 : FilteredComplex