1. Circles in Aztec Diamonds. Aztec Diamonds in Groves. Circles in Groves? Limiting Behavior of Combinatorial Models

2. Circles in Aztec Diamonds An Aztec diamond of order n is defined as the union of those lattice squares whose interiors lie inside the region {(x,y) : |x+y|<= n+1}.

3. Circles in Aztec Diamonds An Aztec diamond of order n is defined as the union of those lattice squares whose interiors lie inside the region {(x,y) : |x+y|<= n+1}. A domino tiling of an Aztec diamond is a way to cover the region with 2 by 1 rectangles (dominoes) so that none of the dominoes overlap, and none of the dominoes extend outside the boundary of the region.

4. Circles in Aztec Diamonds An Aztec diamond of order n is defined as the union of those lattice squares whose interiors lie inside the region {(x,y) :|x+y|<= n+1}. A domino tiling of an Aztec diamond is a way to cover the region with 2 by 1 rectangles (dominoes) so that none of the dominoes overlap, and none of the dominoes extend outside the boundary of the region.

5. Circles in Aztec Diamonds The number of domino tilings of an Aztec diamond is 2^(n(n+1)/2). Any of these tilings can be generated uniformly at random by a procedure called domino shuffling described in a paper of Elkies, Kuperberg, Larsen, and Propp.

6. Circles in Aztec Diamonds Shuffling:

7. Circles in Aztec Diamonds Shuffling: 1. Slide dominoes

8. Circles in Aztec Diamonds Shuffling: 1. Slide dominoes 2. Fill in randomly

9. Circles in Aztec Diamonds Shuffling: 1. Slide dominoes

10. Circles in Aztec Diamonds Shuffling: 1. Slide dominoes 2. Fill in randomly

11. Circles in Aztec Diamonds Shuffling: 0. Delete bad blocks

12. Circles in Aztec Diamonds Shuffling: 0. Delete bad blocks 1. Slide dominoes

13. Circles in Aztec Diamonds Shuffling: 0. Delete bad blocks 1. Slide dominoes 2. Fill in randomly

14. Circles in Aztec Diamonds Shuffling: 0. Delete bad blocks

15. Circles in Aztec Diamonds Shuffling: 0. Delete bad blocks 1. Slide dominoes

16. Circles in Aztec Diamonds Shuffling: 0. Delete bad blocks 1. Slide dominoes 2. Fill in randomly

17. Circles in Aztec Diamonds A domino is called North-going if it migrates north under shuffling, similarly for south, east, and west.

18. Circles in Aztec Diamonds Equivalently, they may be defined by the checkerboard coloring of the plane. A north going domino has a black square on the left and a white square on the right. A south going domino has a white square on the left and a black square on the right. Similarly for east and west.

19. Circles in Aztec Diamonds Equivalently, they may be defined by the checkerboard coloring of the plane. A north going domino has a black square on the left and a white square on the right. A south going domino has a white square on the left and a black square on the right. Similarly for east and west.

20. Circles in Aztec Diamonds We typically color the tiles red, yellow, blue, and green.

21. Circles in Aztec Diamonds A domino is called frozen if it can never be annihilated by further shuffling.

22. Circles in Aztec Diamonds The Arctic Circle Theorem (Jockusch, Propp, Shor): As n (the order of the Aztec diamond) goes to infinity, the expected shape of the boundary between the frozen region and temperate zone is a circle.

23. Circles in Aztec Diamonds The Arctic Circle Theorem (Jockusch, Propp, Shor): Examine the growth model on Young diagrams where each growth position has independent probability ½ of adding a box. This has limiting shape of a quarter-circle (suitably scaled).

24. Circles in Aztec Diamonds Further Statistics of Aztec diamonds: (Cohn, Elkies, and Propp) – Expectations within the temperate zone

25. Circles in Aztec Diamonds Further Statistics of Aztec diamonds: (Johansson) – Fluctuations about the circle. The method of non-intersecting paths, or Brownian motion model yields a link to random matrices and Tracy-Widom distribution. Johansson ultimately equated this model to the random growth model for the Young diagram.

26. Aztec Diamonds in Groves Aztec diamonds can be enumerated by the octahedron recurrence. Let f(n) = the number of Aztec diamonds of order n. Then f(n)f(n-2) = 2f(n-1)^2. f(1) = 2f(2) = 8f(3) = (2f(2)^2)/f(1) = 64f(4) = (2f(3)^2)/f(2) = 1024

27. Aztec Diamonds in Groves Polynomial version of octahedron recurrence: f(i,j,k)f(i,j,k-2) = f(i-1,j,k-1)f(i+1,j,k-1)+f(i,j-1,k-1)f(i,j+1,k- 1) where f(i,j,k) = x(i,j,k) if k=0,-1. Otherwise f(i,j,n) encodes all the tilings of an Aztec diamond of order n. The rational functions that are generated are not just rational in the x(i,j,k), they are Laurent polynomials.

28. Aztec Diamonds in Groves Octahedron Recurrence: f(i,j,k)f(i,j,k-2) = f(i-1,j,k-1)f(i+1,j,k-1)+f(i,j-1,k-1)f(i,j+1,k-1) > f(0,0,2); x(0, 0, 0) x(2, 0, 0) x(-2, 0, 0) x(2, 0, 0) x(-1, 1, 0) x(-1, -1, 0) --------------------------------- + ----------------------------------- x(1, 0, -1) x(-1, 0, -1) x(1, 0, -1) x(-1, 0, -1) x(1, 1, 0) x(1, -1, 0) x(-2, 0, 0) x(1, 1, 0) x(1, -1, 0) x(-1, 1, 0) x(-1, -1, 0) + ---------------------------------- + ----------------------------------------------- x(1, 0, -1) x(-1, 0, -1) x(0, 0, 0) x(1, 0, -1) x(-1, 0, -1) x(1, 1, 0) x(-1, 1, 0) x(1, -1, 0) x(-1, -1, 0) x(1, 1, 0) x(-1, 1, 0) x(0, -2, 0) + ----------------------------------------------- + ---------------------------------- x(0, 0, 0) x(0, 1, -1) x(0, -1, -1) x(0, 1, -1) x(0, -1, -1) x(0, 2, 0) x(1, -1, 0) x(-1, -1, 0) x(0, 0, 0) x(0, 2, 0) x(0, -2, 0) + ----------------------------------- + --------------------------------- x(0, 1, -1) x(0, -1, -1) x(0, 1, -1) x(0, -1, -1) +=++ ++++

29. Aztec Diamonds in Groves Cube Recurrence: f(i,j,k)f(i-1,j-1,k-1) = f(i-1,j,k)f(i,j-1,k-1)+f(i,j-1,k)f(i-1,j,k-1)+f(i-1,j-1,k)f(i,j,k-1) The cube recurrence is a generalization of the octahedron recurrence. As shown by Fomin and Zelevinsky using cluster algebra methods, it also produces Laurent polynomials. But what do the polynomials encode?

30. Aztec Diamonds in Groves Cube Recurrence: f(i,j,k)f(i-1,j-1,k-1) = f(i-1,j,k)f(i,j-1,k-1)+f(i,j-1,k)f(i-1,j,k-1)+f(i-1,j-1,k)f(i,j,k-1) = ??

31. Aztec Diamonds in Groves Cube Recurrence: f(i,j,k)f(i-1,j-1,k-1) = f(i-1,j,k)f(i,j-1,k-1)+f(i,j-1,k)f(i-1,j,k-1)+f(i-1,j-1,k)f(i,j,k-1) = Groves

32. Aztec Diamonds in Groves A grove is a new combinatorial object, due to Carroll and Speyer, given by the cube recurrence. Groves can be viewed as forests that live on a very special planar region or more intuitively, on a three dimensional surface with lattice point corners (- a big pile of cubes). What the surface looks like is specified by some initial conditions. Trivial initial conditions

33. Aztec Diamonds in Groves A grove is a new combinatorial object given by the cube recurrence. Groves can be viewed as forests that live on a very special planar region, given by some specified initial conditions. However, note that the forests have severely restricted behavior. Trivial initial conditions  Unique grove on trivial initials

34. Aztec Diamonds in Groves A grove is a new combinatorial object given by the cube recurrence. Groves can be viewed as forests that live on a very special planar region, given by some specified initial conditions. However, note that the forests have severely restricted behavior. Trivial initial conditions  Unique grove on trvial initials  The grove

35. Aztec Diamonds in Groves A grove is a new combinatorial object given by the cube recurrence. Groves can be viewed as forests that live on a very special planar region, given by some specified initial conditions. However, note that the forests have severely restricted behavior. Kleber initial conditions (4,2,3) Random grove on KI(4,2,3) The grove

36. Aztec Diamonds in Groves A grove is a new combinatorial object given by the cube recurrence. Groves can be viewed as forests that live on a very special planar region, given by some specified initial conditions. However, note that the forests have severely restricted behavior. Aztec diamond initial conditions of order 4 Random grove on AD(4) The grove

37. Aztec Diamonds in Groves Octahedron Recurrence: f(i,j,k)f(i,j,k-2) = f(i-1,j,k-1)f(i+1,j,k-1)+f(i,j-1,k-1)f(i,j+1,k-1) Cube Recurrence: f(i,j,k)f(i-1,j-1,k-1) = f(i-1,j,k)f(i,j-1,k-1)+f(i,j-1,k)f(i-1,j,k-1)+f(i-1,j-1,k)f(i,j,k-1) Remember that the octahedron recurrence is a special case of the cube recurrence.

38. Aztec Diamonds in Groves There is a correspondence between tilings of Aztec diamonds of order n and certain groves on Aztec initial conditions of order n.

39. Aztec Diamonds in Groves Because the octahedron recurrence is a special case of the cube recurrence, there is actually an injection from the set of tilings of Aztec diamonds into the set of groves on Aztec initial conditions. > f(0,0,2); x(0,0,0) x(2,0,0) x(-2,0,0) x(2,0,0) x(-1,1,0) x(-1,-1,0) --------------------------- + ----------------------------- x(1,0,-1) x(-1,0,-1) x(1,0,-1) x(-1,0,-1) x(1,1,0) x(1,-1,0) x(-2,0,0) x(1,1,0) x(1,-1,0) x(-1,1,0) x(-1,-1,0) + ---------------------------- + --------------------------------------- x(1,0,-1) x(-1,0,-1) x(0,0,0) x(1,0,-1) x(-1,0,-1) x(1,1,0) x(-1,1,0) x(1,-1,0) x(-1,-1,0) x(1,1,0) x(-1,1,0) x(0,-2,0) + --------------------------------------- + ---------------------------- x(0,0,0) x(0,1,-1) x(0,-1,-1) x(0,1,-1) x(0,-1,-1) x(0,2,0) x(1,-1,0) x(-1,-1,0) x(0,0,0) x(0,2,0) x(0,-2,0) + ------------------------------ + ---------------------------- x(0,1,-1) x(0,-1,-1) x(0,1,-1) x(0,-1,-1)

40. Aztec Diamonds in Groves The standard initial conditions for a grove look like the compliment of an upside down Q*Bert board. Standard initial conditions of order 8

41. Aztec Diamonds in Groves A grove on standard initial conditions

42. Aztec Diamonds in Groves Groves on standard initial conditions are better represented in a triangular lattice. Notice that we may ignore the short edges. This representation is called a simplified grove.

43. Aztec Diamonds in Groves Like domino shuffling, there is a way to generate groves on standard initial conditions uniformly at random, called grove shuffling (or cube-popping in general).

44. Aztec Diamonds in Groves Like domino shuffling, there is a way to generate groves on standard initial conditions uniformly at random, called grove shuffling (or cube-popping in general).

45. Aztec Diamonds in Groves Like domino shuffling, there is a way to generate groves on standard initial conditions uniformly at random, called grove shuffling (or cube-popping in general).

46. Aztec Diamonds in Groves Like domino shuffling, there is a way to generate groves on standard initial conditions uniformly at random, called grove shuffling (or cube-popping in general).

47. Aztec Diamonds in Groves Like domino shuffling, there is a way to generate groves on standard initial conditions uniformly at random, called grove shuffling (or cube-popping in general).

48. Aztec Diamonds in Groves Like domino shuffling, there is a way to generate groves on standard initial conditions uniformly at random, called grove shuffling (or cube-popping in general).

49. Aztec Diamonds in Groves Like domino shuffling, there is a way to generate groves on standard initial conditions uniformly at random, called grove shuffling (or cube-popping in general).

50. Aztec Diamonds in Groves Like domino shuffling, there is a way to generate groves on standard initial conditions uniformly at random, called grove shuffling (or cube-popping in general ).

51. Aztec Diamonds in Groves Like domino shuffling, there is a way to generate groves on standard initial conditions uniformly at random, called grove shuffling (or cube-popping in general).

52. Aztec Diamonds in Groves Like domino shuffling, there is a way to generate groves on standard initial conditions uniformly at random, called grove shuffling (or cube-popping in general).

53. Circles in Groves? With grove shuffling we can generate large random groves fairly quickly. Four representations of a randomly generated grove of order 20.

54. Circles in Groves? With grove shuffling we can generate large random groves fairly quickly. Representation of an order 200 grove.

55. Circles in Groves? The most promising method of attacking the grove problem seems to be by projecting down one of the colors in a corner,

56. Circles in Groves? The most promising method of attacking the grove problem seems to be by projecting down one of the colors in a corner, isolating the frozen region, and making the situation look like a Young diagram model with growth probability equal ½.

57. Circles in Groves? The most promising method of attacking the grove problem seems to be by projecting the frozen region down to a Young diagram model with growth probability equal ½. Projection of frozen region of a random grove of order 20 above, Young diagram growth model after 20 growth stages below (p= ½ ).

58. Circles in Groves? The most promising method of attacking the grove problem seems to be by projecting the frozen region down to a Young diagram model with growth probability equal ½. Projection of frozen region of a random grove of order 100 above, Young diagram growth model after 100 growth stages below (p= ½ ).

59. Circles in Groves? Looking at a given grove whose projection is above, the observed probabilities of growth are sometimes zero and sometimes 2/3, but never ½! However, I think that if we can take the weighted probabilities over all groves with this projection, then we will find the total probability is equal to the infinite sum of (1/3)^k, k=1 to infinity. What is this sum? ½.

60. Circles in Groves? Other peculiarities… Non-intersecting paths for groves…

61. Circles in Groves? Any Questions? Comments? Suggestions?