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Making Triangulations 4-connected using Flips Prosenjit Bose, Dana Jansens, Andr e van Renssen, Maria Saumell and Sander Verdonschot Carleton University August 8, 2011 Sander Verdonschot (Carleton University) Making Triangulations

  1. Making Triangulations 4-connected using Flips Prosenjit Bose, Dana Jansens, Andr´ e van Renssen, Maria Saumell and Sander Verdonschot Carleton University August 8, 2011 Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 1 / 21

  2. Flips Replace one diagonal of a quadrilateral with the other Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 2 / 21

  3. Flips Replace one diagonal of a quadrilateral with the other Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 2 / 21

  4. Flips Replace one diagonal of a quadrilateral with the other Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 2 / 21

  5. Flip Graph Vertex for each triangulation Edge if two triangulations differ by one flip Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 3 / 21

  6. Flip Graph Vertex for each triangulation Edge if two triangulations differ by one flip Flip Distance: shortest path in flip graph Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 3 / 21

  7. Flip Graph Connected? Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 4 / 21

  8. Flip Graph Connected? Yes - Wagner (1936) Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 4 / 21

  9. Flip Graph Connected? Yes - Wagner (1936) Diameter? O ( n 2 ) - Wagner (1936) Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 4 / 21

  10. Flip Graph Connected? Yes - Wagner (1936) Diameter? O ( n 2 ) - Wagner (1936) 8 n − 54 - Komuro (1997) 6 n − 30 - Mori et al. (2003) Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 4 / 21

  11. Algorithm Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 5 / 21

  12. Algorithm Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 5 / 21

  13. Algorithm Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 5 / 21

  14. Algorithm Mori et al. Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 6 / 21

  15. Algorithm Mori et al. 4-connected ⇒ Hamiltonian Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 6 / 21

  16. Algorithm Mori et al. n − 4 2 n − 11 4-connected ⇒ Hamiltonian Total: 6 n − 30 Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 6 / 21

  17. Algorithm Mori et al. 3 n − 6 5 n − 4 2 n − 11 4-connected ⇒ Hamiltonian Total: 6 n − 30 5 . 2 n − 24 . 4 Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 6 / 21

  18. Making triangulations 4-connected Separating triangle : 3-cycle whose removal disconnects the graph Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 7 / 21

  19. Making triangulations 4-connected Separating triangle : 3-cycle whose removal disconnects the graph No separating triangles ⇐ ⇒ 4-connected Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 7 / 21

  20. Making triangulations 4-connected Separating triangle : 3-cycle whose removal disconnects the graph No separating triangles ⇐ ⇒ 4-connected Flipping an edge of a separating triangle removes it Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 7 / 21

  21. Making triangulations 4-connected Separating triangle : 3-cycle whose removal disconnects the graph No separating triangles ⇐ ⇒ 4-connected Flipping an edge of a separating triangle removes it Prefer shared edges Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 7 / 21

  22. Upper Bound To prove: #flips ≤ (3 n − 6) / 5 Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 8 / 21

  23. Upper Bound To prove: #flips ≤ (3 n − 6) / 5 Charging scheme: Coin on every edge Pay 5 coins per flip Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 8 / 21

  24. Paying for flips Invariant: Every edge of a separating triangle has a coin Charge the flipped edge Charge all edges that aren’t shared Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 9 / 21

  25. Paying for flips Free edge : edge that is not part of any separating triangle Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 10 / 21

  26. Paying for flips Free edge : edge that is not part of any separating triangle Every vertex of a separating triangle is incident to a free edge inside the triangle Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 10 / 21

  27. Paying for flips Free edge : edge that is not part of any separating triangle Invariant: Every vertex of a separating triangle is incident to a free edge inside the triangle that has a coin Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 10 / 21

  28. Paying for flips Free edge : edge that is not part of any separating triangle Invariant: Every vertex of a separating triangle is incident to a free edge inside the triangle that has a coin Charge all free edges that aren’t needed by other separating triangles Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 10 / 21

  29. Which edges to flip? A deepest separating triangle is contained in the maximum number of separating triangles Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 11 / 21

  30. Which edges to flip? A deepest separating triangle is contained in the maximum number of separating triangles Flip: An arbitrary edge Shared with other separating triangles Not shared with a containing triangle Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 11 / 21

  31. Which edges to flip? A deepest separating triangle is contained in the maximum number of separating triangles Flip: An arbitrary edge Shared with other separating triangles Not shared with a containing triangle Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 11 / 21

  32. Which edges to flip? A deepest separating triangle is contained in the maximum number of separating triangles Flip: An arbitrary edge Shared with other separating triangles Not shared with a containing triangle Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 11 / 21

  33. Which edges to flip? Case 1: No shared edges We can charge: The flipped edge An unshared triangle edge An unshared free edge A superfluous free edge Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 12 / 21

  34. Which edges to flip? Case 2: Shares edges with non-containing triangles We can charge: The flipped edge An unshared triangle edge An unshared free edge A superfluous free edge Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 13 / 21

  35. Which edges to flip? Case 3: Shares one edge with containing triangle We can charge: The flipped edge An unshared triangle edge An unshared free edge A superfluous free edge Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 14 / 21

  36. Lower Bound Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 15 / 21

  37. Lower Bound Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 15 / 21

  38. Lower Bound Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 15 / 21

  39. Lower Bound Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 15 / 21

  40. Lower Bound Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 15 / 21

  41. Lower Bound Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 15 / 21

  42. Lower Bound Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 15 / 21

  43. Lower Bound Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 15 / 21

  44. Lower Bound (3 n − 10) / 5 edge-disjoint separating triangles Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 15 / 21

  45. Summary � 3 n − 6 � Any triangulation can be made 4-connected by flips 5 � 3 n − 10 � There are triangulations where this requires flips 5 Sander Verdonschot (Carleton University) Making Triangulations 4-connected August 8, 2011 16 / 21

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