Session 06 - Expansion, Cuts, and Grids

Status: covered from Heptabase lesson eede0316-fe47-4eec-83ce-0356628ac9c2.

Covered Material

• Edge expansion and Cheeger inequality.
• Ramanujan graphs.
• Bisection width.
• 2D grid bottleneck and planar separator intuition.
• Expansion mismatch as the geometric core of Conjecture 3.

Source Anchors

• 2d-grid-bisection-width.md
• 2d-grid-routing-tightness.md
• expansion-cut-to-syndrome-depth.md
• 2d-local-clifford-syndrome-space-depth-tradeoff.md

Retention Check

Ask the learner to derive the informal bound: expander demand Theta(n) divided by 2D grid cut capacity Theta(sqrt(n)) gives Omega(sqrt(n)) depth.