Example 25.2

We now take $X = E^2$ the closed unit disc in the plane, $A = S^1$ the boundary of $E^2$, $B = \{1\}$ and $f$ to be the constant map from $S^1$ to the singleton set $B$. The adjunction space is obtained by collapsing the boundary of $E^2$ to the single point $B$. The resulting space is $S^2$.

Before discussing further examples we relate this to the push-out construction.