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Spectral Sparsification of Graphs

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Claim: every weighted undirected graph admits a spectral sparsifier with about N log N / ε^2 edges that preserves the Laplacian quadratic form up to 1±ε. Construction samples edges proportional to their effective resistances; computing those uses a low‑dimensional sketch via Johnson–Lindenstrauss projection plus O(log N) Laplacian solves. Deterministic 4N/ε^2 sparsifiers exist but are slow; Kelner–Levin give a one‑pass semi‑streaming sampler with amortized polylog updates.

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  1. 0:00Intro
  2. 0:18Graphs
  3. 1:49Intended Application
  4. 2:22Cut Approximation Benczur-Karger'96
  5. 3:22Example: The Complete Graph
  6. 5:49The Laplacian Matrix
  7. 7:48The Laplacian Quadratic Form
  8. 8:55Cuts and the Quadratic Form
  9. 9:34Spectral Approximation ST'04
  10. 12:16Spectral Graph Theory
  11. 12:37Natural Setting
  12. 15:40Outer Product Expansion
  13. 16:52Original Goal
  14. 17:41More General Goal
  15. 18:20Equivalent General Goal
  16. 19:35Core Problem
  17. 21:05Examples of the Reduction
  18. 22:53Effective Resistance View
  19. 24:07Electrical Intuition
  20. 24:39Matrix Chernoff Bound Rudelson'99
  21. 26:29Resistances are Distances
  22. 27:37Nearly Linear Time
  23. 29:22Actual Algorithm
  24. 30:03Connection to Low Rank Appx.
  25. 31:25Deterministic Solution [Batson-Spielman-S'09]
  26. 31:58The Semi-Streaming Model
  27. 33:15Naïve Approach
  28. 34:56Main Idea
  29. 38:45Properties of Resistance
  30. 39:51Resparsification Algorithm
  31. 42:03Why does this work?
  32. 43:01Summary
  33. 43:20Open Questions
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