Tori

Definition

Tori are surfaces that can be visualized as a doughnut shape, defined mathematically as the product of two circles, usually denoted as $T^2 = S^1 \times S^1$. They are significant in topology for their unique properties and serve as classic examples in various theorems, such as the excision theorem and Mayer-Vietoris sequence, which help in understanding how spaces can be decomposed and studied through simpler components.

5 Must Know Facts For Your Next Test

1. The torus has a genus of 1, meaning it has one 'hole', which distinguishes it from other surfaces like spheres or projective planes.
2. In the context of the Mayer-Vietoris sequence, tori can be used to construct more complex spaces by taking the union of simpler open sets.
3. The excision theorem applies to tori, allowing for simplifications in calculating homology groups by removing certain subspaces without altering the overall structure.
4. Tori have nontrivial fundamental groups, specifically $ ext{pi}_1(T^2) \cong \mathbb{Z} \times \mathbb{Z}$, indicating they are not simply connected and have interesting loop structures.
5. When considering products of tori, such as $T^n$, they illustrate the idea of higher-dimensional topology and the interactions between different dimensions.

Review Questions

• How does the structure of a torus influence its use in the excision theorem?
  • The structure of a torus allows for specific applications of the excision theorem because it has well-defined open sets that can be removed without changing homological properties. When applying excision, we can simplify calculations by focusing on subsets of a torus while retaining essential characteristics. This property is useful when analyzing more complex spaces that include tori as components.
• Discuss how the Mayer-Vietoris sequence utilizes tori to understand the topology of more complicated spaces.
  • The Mayer-Vietoris sequence provides a powerful tool for analyzing topological spaces by breaking them down into simpler pieces. When using tori in this context, we can view a torus as being constructed from two overlapping disks. This construction allows us to apply the sequence to derive relationships between their homology groups and those of the entire space. By leveraging the properties of tori, we gain insights into how complex spaces can be pieced together and understood.
• Evaluate how the properties of tori affect their fundamental group and implications for other areas in algebraic topology.
  • Tori exhibit a fundamental group structure that is richer than simply connected spaces due to their nontrivial loops. The fundamental group of a torus is given by $ ext{pi}_1(T^2) \cong \mathbb{Z} \times \mathbb{Z}$, reflecting two independent cycles. This characteristic impacts various areas of algebraic topology, including covering spaces and homotopy theory, as it demonstrates how even simple shapes can lead to intricate behaviors and relationships among different topological constructs.

Related terms

Homotopy: A concept in topology that studies when two functions can be continuously transformed into each other, providing a framework for understanding topological spaces' structures.

Euler Characteristic: A topological invariant that gives a number representing the shape or structure of a topological space, calculated for tori as 0.

Covering Space: A space that 'covers' another space in a specific way, often used in connection with tori to study their fundamental groups.