If such an \(f\) exists, \(X\) and \(Y\) are called homeomorphic, written \(X \cong Y\).
At least four tankers have been struck and Lloyd’s List Intelligence reported that seaborne traffic had dropped by 80% on Sunday, with little sign of a return as key maritime insurers cancelled cover the next day.
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Definition 4 (Definition: Fundamental Group) Let \(X\) be a topological space and \(x_0 \in X\) a basepoint. A loop based at \(x_0\) is a continuous map \(\gamma: [0,1] \to X\) with \(\gamma(0) = \gamma(1) = x_0\). Two loops \(\gamma, \delta\) are homotopic relative to \(x_0\) (written \(\gamma \simeq \delta\)) if there exists a continuous map \(H: [0,1] \times [0,1] \to X\) such that \[H(s,0) = \gamma(s), \quad H(s,1) = \delta(s), \quad H(0,t) = H(1,t) = x_0\] for all \(s,t \in [0,1]\). This is an equivalence relation; denote the equivalence class of \(\gamma\) by \([\gamma]\).
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那场葬礼之后，他和那边的联系渐渐多了起来。虽然五个兄弟姐妹各自在不同的地方讨生活，但谁家修了房子，谁家孩子要结婚，都会互相打个电话道声喜。
When the clubs met 44 years ago they were in the second tier and had huge debts. With Chelsea reportedly £1.6m in the red, the future of Stamford Bridge was in doubt as property developers hovered. Relegation-threatened Wrexham spent most of the 1980s merely trying to survive.。电影对此有专业解读