The infinitely stacked world \(D^2 \times \mathbb{R}^2\) is the universal cover of Bobs world \(S^1 \times D^2\), as it is simply connected and covers Bobs world (see the definition of universal cover above).
43.宋仁德 青海省玉树州动物疫病预防控制中心推广研究员
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