Singular chain complex

Definition

Definition with coefficients over integers (default, if no coefficients specified)

The singular chain complex (or total singular chain complex, to distinguish it from the normalized singular complex) associated with a topological space $X$ is defined as the following chain complex of abelian groups:

Aspect	Definition
chain groups	For $n<0$ the $n^{th}$ chain group $C_n(X)$ is defined to be zero. For $n\ge 0$, the $n^{th}$ chain group $C_n(X)$ is defined as the group of singular n-chains. This is the free abelian group with generating set $S_n(X)$, the set of singular n-simplices. A singular $n$-simplex, in turn, is defined as a continuous map from the standard simplex to $X$. The upshot is that $C_n(X)$ is the group of formal integer linear combinations of continuous maps from the standard simplex to $X$.
boundary map ${\partial }_n$	For $n<1$, the boundary map ${\partial }_n:C_n(X)\to C_{n-1}(X)$ is the zero map. For $n\ge 1$, the map ${\partial }_n:C_n(X)\to C_{n-1}(X)$ is defined as follows. First, note that since $S_n(X)$ freely generates $C_n(X)$, it suffices to describe what ${\partial }_n$ does to $S_n(X)$, and that description extends uniquely to $C_n(X)$. For an element $f\in S_n(X)$, ${\partial }_{n}f$ is the following element of $C_{n-1}(X)$: it is a signed sum $\sum (-1{)}^{j}f\circ i_j$, where $i_j$ is the inclusion map of the standard $(n-1)$-simplex in the standard $n$-simplex as the $j^{th}$ face, with the ordering of vertices preserved, so $f\circ i_j$ is indeed a singular $(n-1)$-simplex, and the signed sum is a singular $(n-1)$-chain.

Variations

• Augmented singular chain complex
• Normalized singular chain complex
• Relative singular chain complex

Functoriality

On the category of topological spaces

Further information: Singular chain complex functor

The total singular complex is a functor from the category of topological spaces with continuous maps to the category of chain complexes with chain maps. The functor associates to a continuous map $f:X\to Y$ to a map $C_n(f):C_n(X)\to C_n(Y)$ as follows. $C_n(f)$ sends a singular $n$-simplex $\sigma$ to $f\circ \sigma$, and more generally sends $\sum a_{\sigma }\sigma$ to $\sum a_{\sigma }f\circ \sigma$.

On the 2-category of topological spaces

Further information: Singular chain complex 2-functor

Consider the 2-category of topological spaces with continuous maps and homotopies. Then the total singular complex is a 2-functor from this category to the 2-category of chain complexes with chain maps and chain homotopies.

This fact implies in particular that the homology of the total singular complex is homotopy-invariant.