Betti numbers and euler characterstic?

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SUMMARY

The discussion centers on the relationship between the second Betti number of a specific simplicial complex and that of a tetrahedron. It concludes that a simplicial complex formed by 2 simplices adjacent to a single edge is contractible, resulting in a trivial reduced homology and a second Betti number of 0. In contrast, the boundary of a tetrahedron is homotopy equivalent to S^2, which has a second Betti number of 1.

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  • Understanding of simplicial complexes
  • Knowledge of Betti numbers
  • Familiarity with homology and homotopy equivalence
  • Basic concepts of algebraic topology
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Let's say you have some type of simplicial complex that is made only of 2 simplices. What happens if all those 2 simplices are adjacent to a single edge (creating a type of book shape), so that this complex can only be embedded in dimensions 3+? Would this complex have the same 2nd betti number as a tetrahedron?
 
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If I am understanding your description correctly, it seems to be that your book complex is a contractible space, and hence has trivial reduced homology. So the 2nd Betti number is 0.

The tetrahedron is also contractible, so the same thing happens. If you mean the boundary of a tetrahedron, then that's homotopy equivalent to S^2, which has 2nd Betti number 1.
 

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