Proof:

We take $ U = S^n - \{{\bf e}_{n+1}\}$ and
$ V = S^n - \{-{\bf e}_{n+1}\}$
and note that $ U\cap V$ deformation retracts to $ S^{n-1}$. Consider the portion of the Mayer Vietoris sequence

$\displaystyle \begin{CD}
@> >> H_n(U)\oplus H_n(V) @> >> H_n(U\cup V) @>{\delta_n} >> H_{n-1}(U\cap V) @> >> H_{n-1}(U)\oplus H_{n-1}(V) @> >>
\end{CD}$

Since $ U$ and $ V$ are contractible spaces, we get for the case $ n\geq 2$,

$\displaystyle \begin{CD}
0 @> >> H_n(U\cup V) @>{\delta_n} >> H_{n-1}(U\cap V) @> >> 0,
\end{CD}$

and hence $ H_n(S^n)\cong H_{n-1}(S^{n-1})$ ($ n\geq 2$).
By induction the result would follow as soon as we prove it for the case $ n = 1$.
For this case let us take a look at the end of the Mayer Vietoris sequence:

$\displaystyle \begin{CD}
0 @> >> H_1(S^1) @> \delta_1 >> H_0(U\cap V) @>(\kappa...
...\kappa^{\prime\prime}) >> H_0(U)\oplus H_0(V) @> >> H_0(U\cup V) @> >>
\end{CD}$

Since $ \delta_1$ is injective,

$\displaystyle H_1(S^1) \cong$   im$\displaystyle \;\delta_1 =$   ker$\displaystyle \;(\kappa^{\prime}, -\kappa^{\prime\prime}).
$

To understand the map $ (\kappa^{\prime}, -\kappa^{\prime\prime})$ we take a basis of $ H_0(U\cap V)$ consisting of a pair of points $ a \in U$ and $ b \in V$.
The singleton $ \{a\}$ generates $ H_0(U)$ and

$\displaystyle k^{\prime}(ma+nb) = ma + nb = n(b-a) + (m+n)a
$

which is a boundary in $ H_0(U)$ if and only if $ m+n = 0$.
Likewise $ k^{\prime\prime}(ma+nb) = 0$ in $ H_0(V)$ if and only if $ m+n = 0$. Thus the kernel of $ (\kappa^{\prime}, \kappa^{\prime\prime})$ is the infinite cyclic group
generated by the zero chain $ a-b$. Hence we get

$\displaystyle H_1(S^1) \cong \mathbb{Z}.
$

To calculate $ H_n(S^1)$
for $ n\geq 2$
we look at the portion of the Mayer Vietoris sequence

$\displaystyle \begin{CD}
@> >> H_n(U)\oplus H_n(V) @> >> H_n(U\cup V) @>{\delta_n} >> H_{n-1}(U\cap V) @> >>
\end{CD}$

and observe that since $ H_n(U) = H_n(V) = H_{n-1}(U\cap V) = 0$ when $ n\geq 2$,

$\displaystyle H_n(S^1) = \{0\},\quad n\geq 2.
$

nisha 2012-03-20