1. What is Algebraic Topology?
Algebraic topology converts topological problems into algebraic ones. Instead of asking "what shape is this?", we ask "what algebraic invariants does it have?"
2. Homology: Counting Holes
Homology counts the "holes" in a space systematically. The n-th Betti number bₙ counts n-dimensional holes.
What Betti Numbers Count
| Betti # | Counts | Example |
|---|---|---|
| b₀ | Connected components | Two separate circles: b₀ = 2 |
| b₁ | 1D holes (loops that can't shrink) | Torus: b₁ = 2 |
| b₂ | 2D holes (voids, cavities) | Sphere: b₂ = 1 |
| b₁(T³) | = 3 | Three generations! |
3. Cohomology: Dual Perspective
Cohomology is the dual of homology — instead of counting cycles, we study differential forms. Cohomology classes are closed forms modulo exact forms.
The Exterior Derivative
• d²= 0: Applying d twice always gives zero
• Closed: dω = 0 (ω has no "boundary")
• Exact: ω = dα (ω is a "boundary")
• Cohomology: Closed forms that aren't exact
4. De Rham Theorem
The de Rham theorem is a profound connection: differential forms (analysis) encode the same information as homology (topology)!
5. Homology of the 3-Torus
The 3-torus T³ = S¹ × S¹ × S¹ has rich homological structure:
The Three 1-Cycles
The three independent 1-cycles on T³ correspond to going around each of the three S¹ factors:
6. Connection to Z² Framework
Why Algebraic Topology Matters
- • Betti numbers: b₁(T³) = 3 explains three generations
- • Cohomology: Differential forms describe gauge fields
- • De Rham theory: Connects field equations to topology
- • Intersection numbers: Count D-brane intersections
Exercises
- Calculate the Betti numbers of a Klein bottle. (Hint: b₁ = 1)
- Show that χ = b₀ - b₁ + b₂ gives χ = 0 for the torus T².
- Why is b₁(T³) = 3 while b₁(T²) = 2? (Hint: product rule)
- If df = 0 on a simply connected space, show f is constant.
- The sphere S² has b₁ = 0. What does this mean for loops on S²?