Exercise 1. A figure 8 loop consists of two circles touching at a point. Show that a torus can be obtained by attaching a disc onto a figure 8 loop.
8 字形环由两个相交的圆组成。证明可以通过将圆盘附加到 8 字形环上来获得环面。 不懂
(3.15) The 2-torus is built by attaching a square to a figure 8. Since the square is topologically a disc, this is a 2-cell attachment. The boundary of the square (disc) is attached in a more interesting way than the previous examples: its boundary runs along the two loops, $a,b$, of the figure 8 in the order $b^{−1}a^{−1}ba$. A torus is a 2-cell attached to a circle?
不懂