A sphere with four punctures occurring where a knot passes through the surface.
Conway Sphere
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References
Adams, C. C. The Knot Book: An Elementary Introduction to the Mathematical Theory of Knots. New York: W. H. Freeman, p. 94, 1994.Referenced on Wolfram|Alpha
Conway SphereCite this as:
Weisstein, Eric W. "Conway Sphere." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ConwaySphere.html