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§ Focal Point

created 2023-03-28 · last edited 2023-04-04
  • The focal point of a space is a point whose only open nbhd is the whole space.
  • In the sierpiski space (), bottom, the bottom is the focal point.
  • In a local ring, the focal point is given by the maximal ideal (in the prime spectrum, ofc).
  • Given any topological space TTT, consider the cone: (ie, take product with [0,1][0, 1][0,1] and smash all the {0}×∗\{0\} \times *{0}×∗ together).
  • Given any topological space TTT, now build the scone: take the product with the sierpinski space, and smash everything with the closed point. Then, the apex of the cone / the closed point becomes a focal point for the topological space. This can be seen as a "one point focalization".
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