Relative homology ($H_{\ast }(X,A)$)

The relative chain complex is $C_{\ast }(X,A)=C_{\ast }(X)/C_{\ast }(A)$, and its homology is the relative homology $H_{\ast }(X,A)$. A pair $A\subseteq U\subseteq X$ produces a long exact sequence for the triple.

Let $A$ be a nonempty union of disjoint embedded circles in a closed oriented topological surface ${\Sigma }_g$, and let $c$ be the number of components after cutting along them. Excision and Poincare-Lefschetz duality identify the relative homology with the complementary-degree cohomology of the cut surface. Its components have boundary, so the relative groups are free, with the displayed ranks; $H_0$ and higher groups vanish. The rank of the circle classes in $H_1({\Sigma }_g)$ is $|{\pi }_0(A)|+1-c$. For a single circle these groups detect whether its class is zero, but for genus at least two cannot distinguish a disk boundary from an essential separating circle.

A relative homology class is represented by a singular chain whose boundary lies in $A$. Two representatives give the same class when their difference is a boundary operator image plus a chain in $A$. Thus a path with endpoints in $A$ can represent a class even when its endpoints are different.

For a subspace $A\subseteq X$, the relative chain complex is the quotient group in each degree,with boundary operator $[c]\mapsto [\partial c]$. This is well defined because $C_{\ast }(A)$ is a chain subcomplex of $C_{\ast }(X)$.

A relative cycle for $A\subset B$ is a chain $b\in C_n(B)$ whose boundary lies in $A$. Equivalently its coset is a homological cycle in $C_{\ast }(B,A)$. Unlike an absolute cycle, its boundary need not be zero. It represents a relative homology class.

If $L$ is a simplicial subcomplex of $K$, thenIts homology is the simplicial relative homology $H_k(K,L)$.

For a pair $A\subseteq X$, the short exact sequence of chain complexes $0\to C_{\ast }(A)\to C_{\ast }(X)\to C_{\ast }(X,A)\to 0$ induces

For $n\ge 1$,In the relative simplicial chain complex, every proper face vanishes, leaving one generator in degree $n$ and zero chain groups in all other degrees.

A pair $(X,A)$ is good when $A$ is closed and is a deformation retract of some neighborhood in $X$. Such a pair satisfies the hypotheses needed to compare relative homology with the reduced homology of a quotient.

For a good pair $(X,A)$, the quotient map induces natural isomorphisms

A simple closed curve in a closed orientable surface has an annular collar neighbourhood, so its collapse is governed by the collapsing a pair theorem. In the long exact sequence in relative homology, the decisive map is $H_1(A;\mathbb{Z})\to H_1({\Sigma }_g;\mathbb{Z})$, sending the circle generator to its homology class. That class is zero for a separating curve and a primitive homology class for a nonseparating curve. The two possibilities lead to homology after collapsing a separating surface curve and homology after collapsing a nonseparating surface curve.

For genus $g\ge 1$, collapse a nonseparating simple closed curve on a closed orientable surface. Its homology class is primitive, as a curve meeting it transversely once detects by the intersection pairing on an oriented surface. The long exact sequence in relative homology therefore gives $H_0=\mathbb{Z}$, $H_1={\mathbb{Z}}^{2g-1}$, $H_2=\mathbb{Z}$, and zero higher groups for the quotient. Geometrically it has the homotopy type ${\Sigma }_{g-1}\vee S^1$.

Collapse a separating simple closed curve on a closed orientable surface of genus $g$. If its two sides have genera $g_1,g_2$, the quotient is homeomorphic to ${\Sigma }_{g_1}\vee {\Sigma }_{g_2}$. Its integral homology is $\mathbb{Z}$ in degree zero, ${\mathbb{Z}}^{2g}$ in degree one, ${\mathbb{Z}}^2$ in degree two, and zero above degree two. In relative homology, the circle maps to zero in $H_1({\Sigma }_g)$, and the second relative group fits into a split sequence $0\to \mathbb{Z}\to H_2({\Sigma }_g,A)\to \mathbb{Z}\to 0$. The two collapsed sides retain independent fundamental classes.