Gianluca
... 4. All right angles are equal 5. If a line crossing two other lines makes the interior angles on the same side less than two right angles, then these two lines will meet on that side when extended far enough Again, as we already saw in section 3 (p. 39), assuming all the first four axioms, we actual ...
... 4. All right angles are equal 5. If a line crossing two other lines makes the interior angles on the same side less than two right angles, then these two lines will meet on that side when extended far enough Again, as we already saw in section 3 (p. 39), assuming all the first four axioms, we actual ...
3-manifold
In mathematics, a 3-manifold is a space that locally looks like Euclidean 3-dimensional space. Intuitively, a 3-manifold can be thought of as a possible shape of the universe. Just like a sphere looks like a plane to a small enough observer, all 3-manifolds look like our universe does to a small enough observer. This is made more precise in the definition below.