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| | If It Looks Like a Sphere...: Science News Online, June 14, 2003 |
 | | Even though spheres and tori sit in three-dimensional space, mathematicians focus on their surfaces and so view them as two-dimensional, unlike solid balls and filled-in doughnuts, which are three-dimensional. |
 | | For instance, just as the sphere is the two-dimensional boundary of the three-dimensional ball, mathematicians have defined the hypersphere as the three-dimensional boundary of the four-dimensional ball—a space that's hard to visualize but that can nevertheless be analyzed mathematically. |
 | | In the late 1970s, mathematician William Thurston, now at the University of California, Davis, envisioned a way to tame the menagerie of three-dimensional spaces—an idea that gave mathematicians a roadmap for proving the Poincaré conjecture. |
| www.sciencenews.org /articles/20030614/bob10.asp (211 words) |
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