A filled torus (a doughnut) is a 3-manifold homeomorphic to \(S^1 \times D^2\), where \(D^2\) is the 2-dimensional disk. There exists a deformation retract from the doughnut to a circle, so the fundamental group of the doughnut is \(\pi_1(S^1 \times D^2) \cong \mathbb{Z}\).
Студенты нашли останки викингов в яме для наказаний14:52。业内人士推荐旺商聊官方下载作为进阶阅读
Looking for a new television, but don't want to break the bank on fancy QD-OLED technology and features you may not need? This 55-inch screen from Vizio is marked down to $237 at Walmart, a 10 percent discount from its usual, already quite low price. It would make a great second TV for your garage or guest room.，更多细节参见体育直播
圖像來源，Hulton Archive via Getty Images