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| | Erlangen program Summary |
 | | For example, in the plane, a rotation (say, a quarter-turn) about a fixed point is a transformation, since it maps the plane onto itself, and no two different points get rotated to the same point. |
 | | On the other hand, a map that sends the plane onto the upper-half plane by folding it along the x-axis like a sheet of paper is not a transformation, since it does not map the plane onto the entire plane, and two different points can get mapped to the same point. |
 | | The transformations that preserve Euclidean space are the rotations, translations (maps that shift the plane in some fixed direction, by some fixed amount), reflections across a line, and glide reflections (maps that consist of a reflection across a line, followed by a shift in the direction of the line). |
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