Wedge of two circles:

Let us consider the space $ S^1\vee S^1$ given by the union of two circles of radius one in the plane touching each other externally at the origin. We take $ U$ and $ V$ to be the open sets obtained by deleting one of the points of each lobe (not the common point!). Then the circle is a deformation retract of both $ U$ and $ V$ and $ U\cap V$ deformation retracts to the origin. Thus

$\displaystyle \pi_1(S^1\vee S^1) = \mathbb{Z}*\mathbb{Z}. \eqno(26.3)
$

The last clause in theorem (26.1) also provides the generators of the fundamental group. Assuming the circles to centered at $ \pm 1$, the generators are given by the homotopy classes of the loops

$\displaystyle \pm 1 + \exp(2\pi i t) \eqno(26.4)
$

The generalization to a wedge of $ n$ circles is left as an exercise.

nisha 2012-03-20