<Chapter><Heading>Chain Complexes</Heading>
HAP uses implementations of chain complexes of free <M>\mathbb K</M>-modules for each of the rings  <M>\mathbb K = \mathbb Z</M>, <M>\mathbb K = \mathbb Q</M>, <M>\mathbb K = \mathbb F_p</M> with <M>p</M> a prime number, <M>\mathbb K = \mathbb ZG</M>, <M>\mathbb K = \mathbb F_pG</M> with <M>G</M> a group.
 The implemented chain complexes have the form
<P/><M> C_n \stackrel{d_n}{\longrightarrow } C_{n-1} \stackrel{d_{n-1}}{\longrightarrow } \cdots \stackrel{d_2}{\longrightarrow } C_1 \stackrel{d_1}{\longrightarrow } C_0 \stackrel{d_0}{\longrightarrow } 0\ .</M>
<P/>Such a complex is said to have <E>length</E> <M>n</M>
and the   rank of the free <M>\mathbb K</M>-module <M>C_k</M> is referred to as the <E>dimenion</E> of the complex in degree <M>k</M>. 
<P/> For the case <M>\mathbb K = \mathbb ZG</M>  (resp. <M>\mathbb K = \mathbb F_pG</M>) the main focus is on free chain complexes that are exact at each degree <M>k</M>, i.e.  <M>{\rm im}(d_{k+1})={\rm ker}(d_k)</M>, for <M>0 &lt; k &lt; n</M> and with <M>C_0/{\rm im}(d_1) \cong \mathbb Z</M> (resp. <M>C_0/{\rm im}(d_1) \cong \mathbb F_p</M>). We refer to such a chain complex as a <E>resolution of length </E> <M>n</M> even though  <M>d_n</M> will typically not be injective. More correct terminology  would refer to such a chain complex as the first <M>n</M> degrees of a free resolution.      
<P/>The following sections illustrate some constructions of chain complexes. Constructions for resolutions are described in the next chapter <Ref Sect="resolutions"/>.
<Section><Heading>Chain complex of a simplicial complex and simplicial pair</Heading>

<P/>The following example constructs the Quillen simplicial complex <M>Q={\mathcal A}_p(G)</M> for <M>p=2</M> and <M>G=A_8</M>; this is the order complex of the poset of non-trivial elementary <M>2</M>-subgroups of <M>G</M>. The chain complex <M>C_\ast = C_\ast(Q)</M> is then computed and seen to have the same number of free generators as <M>Q</M> has simplices.
(To ensure indexing of subcomplexes is consistent with that of the large complex it is best to work with vertices represented as integers.)
<Example>
<#Include SYSTEM "tutex/13.1.txt">
</Example>

Next the simplicial complex <M>Q</M> is converted to one whose vertices are represented by integers and  a contactible subcomplex <M>L &lt; Q</M> is computed.
The chain complex <M>D_\ast=C_\ast(Q,L)</M> of the simplicial pair <M>(Q,L)</M> is constructed and seen to have the correct size. 

<Example>
<#Include SYSTEM "tutex/13.2.txt">
</Example>

The next commands produce a smalled chain complex <M>B_\ast</M> chain homotopy equivalent to <M>D_\ast</M> and compute the homology <M>H_k(Q,\mathbb Z) \cong H_k(B_\ast)</M> for <M>k=1,2,3</M>.

<Example>
<#Include SYSTEM "tutex/13.3.txt">
</Example>


</Section>

<Section><Heading>Chain complex of a cubical complex and cubical pair</Heading>
The following example reads in the digital image 

<P/>
<Alt Only="HTML">&lt;img src="images/bw_image.bmp" align="center" height="300" alt="a digital image"/>
</Alt>


<P/>as a <M>2</M>-dimensional pure cubical complex <M>M</M> and constructs the chain complex <M>C_\ast=C_\ast(M)</M>.

<Example>
<#Include SYSTEM "tutex/13.4.txt">
</Example>

Next an acyclic pure cubical subcomplex  <M>L &lt; M</M> is computed
and the chain complex <M>D_\ast=C_\ast(M,L)</M> of the pair is constructed.

<Example>
<#Include SYSTEM "tutex/13.5.txt">
</Example>

Finally the chain complex <M>D_\ast</M> is simplified to a  homotopy equivalent chain complex <M>B_\ast</M> and the homology <M>H_1(M,\mathbb Z) \cong H_1(B_\ast)</M> is computed.

<Example>
<#Include SYSTEM "tutex/13.6.txt">
</Example>

</Section>

<Section><Heading>Chain complex of a regular CW-complex</Heading>

The next example constructs a <M>15</M>-dimensional regular CW-complex <M>Y</M>
that  is homotopy equivalent to the <M>2</M>-dimensional torus.

<Example>
<#Include SYSTEM "tutex/13.7.txt">
</Example>

Next the cellular chain complex <M>C_\ast=C_\ast(Y)</M> is constructed. Also, a minimally generated  chain complex <M>D_\ast=C_\ast(Y')</M> of a non-regular CW-complex <M>Y'\simeq Y</M> is constructed.

<Example>
<#Include SYSTEM "tutex/13.8.txt">
</Example>

</Section>

<Section><Heading>Chain Maps of simplicial and regular CW maps</Heading>

The next example realizes the complement of the first prime knot on <M>11</M> crossings as a pure permutahedral complex. The complement is converted to a 
regular CW-complex <M>Y</M> and the
  boundary inclusion <M>f\colon \partial Y \hookrightarrow Y</M> is constructed as a map of regular CW-complexes. Then the induced chain map <M>F\colon C_\ast(\partial Y) \hookrightarrow C_\ast(Y)</M> is constructed. Finally the homology homomorphism <M>H_1(F)\colon H_1(C_\ast(\partial Y)) \rightarrow H_1(C_\ast(Y))</M> is computed.

<Example>
<#Include SYSTEM "tutex/13.9.txt">
</Example>

The command <Code>ChainMap(f)</Code> can be used to construct the chain map
<M>C_\ast(K) \rightarrow C_\ast(K')</M> induced by a map <M>f\colon K\rightarrow K'</M> of simplicial complexes. 

</Section>

<Section><Heading>Constructions for chain complexes</Heading>
It is straightforward to implement basic constructions on chain complexes.
A few constructions are illustrated in the following example.

<Example>
<#Include SYSTEM "tutex/13.10.txt">
</Example>
</Section>

<Section><Heading>Filtered chain complexes</Heading>

A sequence of inclusions of chain complexes
<M>C_{0,\ast} \le C_{1,\ast} \le \cdots \le C_{T-1,\ast} \le C_{T,\ast}</M> in which the preferred basis of <M>C_{k-1,\ell}</M> is the beginning of the preferred basis of <M>C_{k,\ell}</M> is referred to as a <E>filtered chain complex</E>.
Filtered chain complexes give rise to spectral sequences such as
 the <E>equivariant spectral sequence</E> of a <M>G-CW</M>-complex with subgroup <M>H &lt; G</M>. A particular case is the  
 Lyndon-Hochschild-Serre spectral sequence for the homology of a group extension <M>N \rightarrowtail G \twoheadrightarrow Q</M> with <M>E^2_{p,q}=H_p(Q,H_q(N, \mathbb Z))</M>.

<P/>The following commands construct the filtered chain complex underlying the Lyndon-Hochschild-Serre spectral sequence for the dihedral group <M>G=D_{32}</M> of order 64 and its centre <M>N=Z(G)</M>.

<Example>
<#Include SYSTEM "tutex/13.11.txt">
</Example>

The differentials <M>d^r_{p,q}</M> in a given page <M>E^r</M> of
the spectral sequence arise from the induced homology homomorphisms <M>\iota^{s,t}_\ell\colon H_{\ell}(C_{s,\ast}) \rightarrow H_{\ell}(C_{t,\ast})</M> for <M>s\le t</M>. Textbooks traditionally picture the differential in <M>E^r</M> as an array of sloping arrows with non-zero groups <M>E^r_{p,q}\neq 0</M> represented by dots.  An alternative representation of this information is as a barcode (of
the sort used in 
Topological Data Analysis). The  homomorphisms <M>\iota^{\ast,\ast}_2</M>
in the example, with coefficients converted to mod <M>2</M>,  are pictured by the bar code
<P/>
<Alt Only="HTML">&lt;img src="images/lhsbc.png" align="center" height="150" alt="a bar code for the LHS spectral sequence"/>
</Alt>
<P/>
which was produced by the following commands.

<Example>
<#Include SYSTEM "tutex/13.12.txt">
</Example>
</Section>

<Section><Heading>Sparse chain complexes</Heading>
Boundary homomorphisms in all of the above examples of
chain complexes are represented by matrices. In cases where the matrices are large and have many zero entries it is better to use sparse matrices.

<P/>The following commands demonstrate the conversion of the matrix
<P/><M>A=\left(\begin{array}{ccc}
0 &amp;2 &amp;0\\
-3 &amp;0 &amp; 0\\
0 &amp; 0 &amp;4
\end{array}\right)</M>
<P/>to sparse form, and vice-versa. 
<Example>
<#Include SYSTEM "tutex/13.13.txt">
</Example>

<P/>To illustrate the use of sparse chain complexes we consider the data points represented in the following digital image.
<P/><Alt Only="HTML">&lt;img src="images/data500.png" align="center" height="200" alt="data points samples from an annulus"/>
</Alt>
<P/>
The following commands read in this image as a <M>2</M>-dimensional pure cubical complex and store the Euclidean coordinates of the black pixels in a list. Then 200 points are selected at random from this list and used to construct a  <M>200\times 200</M> symmetric matrix <M>S</M> whose entries are the Euclidean distance between the sample data points.


<Example>
<#Include SYSTEM "tutex/13.14.txt">
</Example>

The symmetric distance matrix <M>S</M> is next converted to a filtered chain complex arising from a filtered simplicial complex (using the standard <E>persistent homology</E> pipeline).
<Example>
<#Include SYSTEM "tutex/13.15.txt">
</Example>

Next, the induced homology homomorphisms in degrees 1 and 2, with rational coefficients, are computed and displayed a barcodes.
<Example>
<#Include SYSTEM "tutex/13.15a.txt">
</Example>
<P/><Alt Only="HTML">&lt;img src="images/barcode0example.png" align="center" height="130" alt="Degree 0 persistent homology barcode"/>
</Alt>
<P/>
<Example>
<#Include SYSTEM "tutex/13.16.txt">
</Example>
<P/><Alt Only="HTML">&lt;img src="images/barcode1example.png" align="center" height="130" alt="Degree 1 persistent homology barcode"/>
</Alt>
<P/>
The barcodes are consistent with the data points having been sampled from a space with the homotopy type of an annulus.
</Section>
</Chapter>