Computing homology group using Mayer-Vietoris sequence

Question

Suppose I am given an exact sequence:

$$0\to G\xrightarrow{f} \mathbb{Z} \xrightarrow{g} \mathbb{Z} \xrightarrow{h} H\to 0 $$

where the first $\mathbb{Z}=H_3(A\cup B)$ and the second $\mathbb{Z}=H_2(A\cap B)$ where $A,B$ are solid tori. I compute the homology of $A\cup B$ and $A \cap B$ by noting that $A\cup B=S^3, A\cap B=$ Torus.

I know by exactness that $f$ should be injective, i.e. $ker(f)=0$ and $h$ should be surjective, i.e. $im(h)=H$. But I got stuck when computing $G$ and $H$.

Any help is appreciated :)

Answer 1

From a short exact sequence $$0\rightarrow A\rightarrow B\rightarrow C\rightarrow 0,$$ which is easier than the case you describe, knowing $A$ and $B$ in general is not enough to compute $C$. This is the extension problem.

Some examples $$ 0\rightarrow \mathbb{Z}\rightarrow \mathbb{Z}\rightarrow 0\rightarrow 0, $$

or $$ 0\rightarrow \mathbb{Z}\rightarrow \mathbb{Z}\rightarrow \mathbb{Z}/n\mathbb{Z}\rightarrow 0.$$

You should know some more information of the maps in the sequence to go further with your computation.