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 \(\pi_1(S^1 \times D^2) \cong \mathbb{Z}\).
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Конфликт США с Ираном назвали ударом для Украины14:58
重点督办一批建议，并不意味着不办理其他建议。。关于这个话题，体育直播提供了深入分析