group walk random graphs - university of bristolmb13434/pr_talks/... · agelos georgakopoulos...
TRANSCRIPT
![Page 1: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/1.jpg)
Group Walk Random Graphs
Agelos Georgakopoulos
Bristol, 13/2/17
Partly joint work with J. Haslegraveand with V. Kaimanovich
Agelos Georgakopoulos
![Page 2: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/2.jpg)
Geometric Random Graphs Literature
[Remco Van Der Hofstad. Random graphs and complex networks. LectureNotes, 2013.]
[Mathew Penrose. Random Geometric Graphs. Oxford University Press,2003.]
Random planar graphs ...
Percolation theory ...
Agelos Georgakopoulos
![Page 3: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/3.jpg)
Geometric Random Graphs Literature
[Remco Van Der Hofstad. Random graphs and complex networks. LectureNotes, 2013.]
[Mathew Penrose. Random Geometric Graphs. Oxford University Press,2003.]
Random planar graphs ...
Percolation theory ...
Agelos Georgakopoulos
![Page 4: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/4.jpg)
How Mafia’s grow
A network evolves in (continuous or discrete) time with thefollowing rules:
When a (Poisson) clock ticks, nodes split into two;When a node x splits into two nodes x ′, x ′′, each of itsexisting edges gets inherited by x ′ or x ′′ independentlywith probability 1/2;Moreover, a Poisson(k)-distributed number of new edgesare added between x ′ and x ′′.
As time goes to infinity, the distribution of the component(mafia) of a designated vertex converges.
Is the component in the limit distribution finite or infinite?If it is finite, is its expected size finite or infinite?
If finite, how does it depend on k?
Agelos Georgakopoulos
![Page 5: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/5.jpg)
How Mafia’s grow
A network evolves in (continuous or discrete) time with thefollowing rules:
When a (Poisson) clock ticks, nodes split into two;When a node x splits into two nodes x ′, x ′′, each of itsexisting edges gets inherited by x ′ or x ′′ independentlywith probability 1/2;Moreover, a Poisson(k)-distributed number of new edgesare added between x ′ and x ′′.
As time goes to infinity, the distribution of the component(mafia) of a designated vertex converges.
Is the component in the limit distribution finite or infinite?If it is finite, is its expected size finite or infinite?
If finite, how does it depend on k?
Agelos Georgakopoulos
![Page 6: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/6.jpg)
How Mafia’s grow
A network evolves in (continuous or discrete) time with thefollowing rules:
When a (Poisson) clock ticks, nodes split into two;When a node x splits into two nodes x ′, x ′′, each of itsexisting edges gets inherited by x ′ or x ′′ independentlywith probability 1/2;Moreover, a Poisson(k)-distributed number of new edgesare added between x ′ and x ′′.
As time goes to infinity, the distribution of the component(mafia) of a designated vertex converges.
Is the component in the limit distribution finite or infinite?
If it is finite, is its expected size finite or infinite?If finite, how does it depend on k?
Agelos Georgakopoulos
![Page 7: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/7.jpg)
How Mafia’s grow
A network evolves in (continuous or discrete) time with thefollowing rules:
When a (Poisson) clock ticks, nodes split into two;When a node x splits into two nodes x ′, x ′′, each of itsexisting edges gets inherited by x ′ or x ′′ independentlywith probability 1/2;Moreover, a Poisson(k)-distributed number of new edgesare added between x ′ and x ′′.
As time goes to infinity, the distribution of the component(mafia) of a designated vertex converges.
Is the component in the limit distribution finite or infinite?If it is finite, is its expected size finite or infinite?
If finite, how does it depend on k?
Agelos Georgakopoulos
![Page 8: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/8.jpg)
How Mafia’s grow
A network evolves in (continuous or discrete) time with thefollowing rules:
When a (Poisson) clock ticks, nodes split into two;When a node x splits into two nodes x ′, x ′′, each of itsexisting edges gets inherited by x ′ or x ′′ independentlywith probability 1/2;Moreover, a Poisson(k)-distributed number of new edgesare added between x ′ and x ′′.
As time goes to infinity, the distribution of the component(mafia) of a designated vertex converges.
Is the component in the limit distribution finite or infinite?If it is finite, is its expected size finite or infinite?
If finite, how does it depend on k?
Agelos Georgakopoulos
![Page 9: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/9.jpg)
Random Graphs from trees
R13(T )
Agelos Georgakopoulos
![Page 10: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/10.jpg)
Random Graphs from trees
Simulations by C. Moniz.
Further examples. Here are some more random graphs Rk,n outputted by my algo-
rithm for di↵erent values of k and n:
12
3
4 5
6
7
8
910 11
12
1314
15
16
17
1819
20
2122
23 24
25
26
2728
29
30
3132
3334
35
36
3738
39
40
41
42
4344
4546
4748
4950 5152
53 545556
57 58
5960
6162
63
64
Figure 3: Rk,n obtained from binary trees, for di↵erent values of k and n. From top-leftto bottom-right, the values are k = 3 and n = 6, k = 3 and n = 8, k = 6 and n = 10,k = 7 and n = 10, respectively. Drawn using Python igraph [20].
Notice that while the graph is connected for k = 3 walks on a tree of depth n = 6, it is
disconnected for k = 3 walks on a tree of depth n = 8. The example of R3,8 illustrated in
figure 3 has 15 connected components.
One may be interested in how often the random graph obtained from this construction is
connected, for di↵erent values of k and n. In the case of n = 10, I observed (informally)
that more often than not, a graph with k = 6 would be disconnected and a graph with
k = 7 walkers would be connected. Some examples of this are illustrated in figure 3.
19
Further examples. Here are some more random graphs Rk,n outputted by my algo-
rithm for di↵erent values of k and n:
12
3
4 5
6
7
8
910 11
12
1314
15
16
17
1819
20
2122
23 24
25
26
2728
29
30
3132
3334
35
36
3738
39
40
41
42
4344
4546
4748
4950 5152
53 545556
57 58
5960
6162
63
64
Figure 3: Rk,n obtained from binary trees, for di↵erent values of k and n. From top-leftto bottom-right, the values are k = 3 and n = 6, k = 3 and n = 8, k = 6 and n = 10,k = 7 and n = 10, respectively. Drawn using Python igraph [20].
Notice that while the graph is connected for k = 3 walks on a tree of depth n = 6, it is
disconnected for k = 3 walks on a tree of depth n = 8. The example of R3,8 illustrated in
figure 3 has 15 connected components.
One may be interested in how often the random graph obtained from this construction is
connected, for di↵erent values of k and n. In the case of n = 10, I observed (informally)
that more often than not, a graph with k = 6 would be disconnected and a graph with
k = 7 walkers would be connected. Some examples of this are illustrated in figure 3.
19
Agelos Georgakopoulos
![Page 11: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/11.jpg)
Random Graphs from trees
Simulations by C. Moniz.
Further examples. Here are some more random graphs Rk,n outputted by my algo-
rithm for di↵erent values of k and n:
12
3
4 5
6
7
8
910 11
12
1314
15
16
17
1819
20
2122
23 24
25
26
2728
29
30
3132
3334
35
36
3738
39
40
41
42
4344
4546
4748
4950 5152
53 545556
57 58
5960
6162
63
64
Figure 3: Rk,n obtained from binary trees, for di↵erent values of k and n. From top-leftto bottom-right, the values are k = 3 and n = 6, k = 3 and n = 8, k = 6 and n = 10,k = 7 and n = 10, respectively. Drawn using Python igraph [20].
Notice that while the graph is connected for k = 3 walks on a tree of depth n = 6, it is
disconnected for k = 3 walks on a tree of depth n = 8. The example of R3,8 illustrated in
figure 3 has 15 connected components.
One may be interested in how often the random graph obtained from this construction is
connected, for di↵erent values of k and n. In the case of n = 10, I observed (informally)
that more often than not, a graph with k = 6 would be disconnected and a graph with
k = 7 walkers would be connected. Some examples of this are illustrated in figure 3.
19
Further examples. Here are some more random graphs Rk,n outputted by my algo-
rithm for di↵erent values of k and n:
12
3
4 5
6
7
8
910 11
12
1314
15
16
17
1819
20
2122
23 24
25
26
2728
29
30
3132
3334
35
36
3738
39
40
41
42
4344
4546
4748
4950 5152
53 545556
57 58
5960
6162
63
64
Figure 3: Rk,n obtained from binary trees, for di↵erent values of k and n. From top-leftto bottom-right, the values are k = 3 and n = 6, k = 3 and n = 8, k = 6 and n = 10,k = 7 and n = 10, respectively. Drawn using Python igraph [20].
Notice that while the graph is connected for k = 3 walks on a tree of depth n = 6, it is
disconnected for k = 3 walks on a tree of depth n = 8. The example of R3,8 illustrated in
figure 3 has 15 connected components.
One may be interested in how often the random graph obtained from this construction is
connected, for di↵erent values of k and n. In the case of n = 10, I observed (informally)
that more often than not, a graph with k = 6 would be disconnected and a graph with
k = 7 walkers would be connected. Some examples of this are illustrated in figure 3.
19
Agelos Georgakopoulos
![Page 12: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/12.jpg)
A nice property
Agelos Georgakopoulos
![Page 13: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/13.jpg)
A nice property
Proposition
E(] edges xy in Rn
with x in X and y in Y )
converges.
Agelos Georgakopoulos
![Page 14: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/14.jpg)
A nice property
Proposition
E(] edges xy in Rn
with x in X and y in Y )
converges.
?
Agelos Georgakopoulos
![Page 15: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/15.jpg)
A nice property
Proposition
E(] edges xy in Rn
with x in X and y in Y )
converges.
?Agelos Georgakopoulos
![Page 16: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/16.jpg)
The Poisson integral representation formula
The classical Poisson formula
h(z) =∫ 1
0h(θ)P(z, θ)dθ
=
∫ 1
0h(θ)dνz(θ)
where P(z, θ) := 1−|z |2
|e2πiθ−z |2 ,recovers every continuous harmonicfunction h on D from its boundaryvalues h on the circle ∂D.
Agelos Georgakopoulos
![Page 17: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/17.jpg)
The Poisson integral representation formula
The classical Poisson formula
h(z) =∫ 1
0h(θ)P(z, θ)dθ =
∫ 1
0h(θ)dνz(θ)
where P(z, θ) := 1−|z |2
|e2πiθ−z |2 ,recovers every continuous harmonicfunction h on D from its boundaryvalues h on the circle ∂D.
Agelos Georgakopoulos
![Page 18: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/18.jpg)
The Poisson-Furstenberg boundary
The Poisson boundary of an (infinite) graph G consists of- a (Lebesgue-Rohlin) measurable space (PG,Σ), and
- a family of probability measures {νz , z ∈ VG},such that
every bounded harmonic function h can be obtained by
h(z) =∫PG
h(η)dνz(η)
this h ∈ L∞(PG) is unique up to modification on a null-set;conversely, for every h ∈ L∞(PG) the functionz 7→
∫PG
h(η)dνz(η) is bounded and harmonic.
i.e. there is Poisson-like formula establishing an isometrybetween the Banach spaces H∞(G) and L∞(PG).
Agelos Georgakopoulos
![Page 19: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/19.jpg)
The Poisson-Furstenberg boundary
The Poisson boundary of an (infinite) graph G consists of- a (Lebesgue-Rohlin) measurable space (PG,Σ), and- a family of probability measures {νz , z ∈ VG},
such that
every bounded harmonic function h can be obtained by
h(z) =∫PG
h(η)dνz(η)
this h ∈ L∞(PG) is unique up to modification on a null-set;conversely, for every h ∈ L∞(PG) the functionz 7→
∫PG
h(η)dνz(η) is bounded and harmonic.
i.e. there is Poisson-like formula establishing an isometrybetween the Banach spaces H∞(G) and L∞(PG).
Agelos Georgakopoulos
![Page 20: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/20.jpg)
The Poisson-Furstenberg boundary
The Poisson boundary of an (infinite) graph G consists of- a (Lebesgue-Rohlin) measurable space (PG,Σ), and- a family of probability measures {νz , z ∈ VG},
such that
every bounded harmonic function h can be obtained by
h(z) =∫PG
h(η)dνz(η)
this h ∈ L∞(PG) is unique up to modification on a null-set;conversely, for every h ∈ L∞(PG) the functionz 7→
∫PG
h(η)dνz(η) is bounded and harmonic.
i.e. there is Poisson-like formula establishing an isometrybetween the Banach spaces H∞(G) and L∞(PG).
Agelos Georgakopoulos
![Page 21: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/21.jpg)
The Poisson-Furstenberg boundary
The Poisson boundary of an (infinite) graph G consists of- a (Lebesgue-Rohlin) measurable space (PG,Σ), and- a family of probability measures {νz , z ∈ VG},
such that
every bounded harmonic function h can be obtained by
h(z) =∫PG
h(η)dνz(η)
this h ∈ L∞(PG) is unique up to modification on a null-set;
conversely, for every h ∈ L∞(PG) the functionz 7→
∫PG
h(η)dνz(η) is bounded and harmonic.
i.e. there is Poisson-like formula establishing an isometrybetween the Banach spaces H∞(G) and L∞(PG).
Agelos Georgakopoulos
![Page 22: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/22.jpg)
The Poisson-Furstenberg boundary
The Poisson boundary of an (infinite) graph G consists of- a (Lebesgue-Rohlin) measurable space (PG,Σ), and- a family of probability measures {νz , z ∈ VG},
such that
every bounded harmonic function h can be obtained by
h(z) =∫PG
h(η)dνz(η)
this h ∈ L∞(PG) is unique up to modification on a null-set;conversely, for every h ∈ L∞(PG) the functionz 7→
∫PG
h(η)dνz(η) is bounded and harmonic.
i.e. there is Poisson-like formula establishing an isometrybetween the Banach spaces H∞(G) and L∞(PG).
Agelos Georgakopoulos
![Page 23: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/23.jpg)
The Poisson-Furstenberg boundary
Selected work on the Poisson boundary
Introduced by Furstenberg to study semi-simpleLie groups [Annals of Math. ’63]Kaimanovich & Vershik give a general criterion using theentropy of random walk [Annals of Probability ’83]Kaimanovich identifies the Poisson boundary of hyperbolicgroups, and gives general criteria [Annals of Math. ’00]
Agelos Georgakopoulos
![Page 24: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/24.jpg)
A nice property
PropositionFor every two measurable
subsets X ,Y of the Poisson (orMartin) boundary ∂G,
E(] edges xy in Rn
with x ‘close to’ Xand y ‘close to’ Y )
converges.
We use the limit to define a measure on ∂G × ∂G via
C(X ,Y ) := limE(] edges ...)
Agelos Georgakopoulos
![Page 25: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/25.jpg)
A nice property
PropositionFor every two measurable
subsets X ,Y of the Poisson (orMartin) boundary ∂G,
E(] edges xy in Rn
with x ‘close to’ Xand y ‘close to’ Y )
converges.
We use the limit to define a measure on ∂G × ∂G via
C(X ,Y ) := limE(] edges ...)
Agelos Georgakopoulos
![Page 26: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/26.jpg)
Energy and Douglas’ formula
The classical Douglas formula [Douglas ’31]
E(h) =∫ 2π
0
∫ 2π
0(h(η) − h(ζ))2Θ(ζ, η)dηdζ
calculates the (Dirichlet) energy of aharmonic function h on D from itsboundary values h on the circle ∂D.
Agelos Georgakopoulos
![Page 27: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/27.jpg)
Energy in finite electrical networks
a
b
a
b
Cab
E(h) =∑
a,b∈B (h(a) − h(b))2 Cab,
Compare with Douglas: E(h) =∫ 2π0
∫ 2π0 (h(η) − h(ζ))2Θ(ζ, η)dηdζ
How can we generalise this to an arbitrary domain?To an infinite graph?
Agelos Georgakopoulos
![Page 28: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/28.jpg)
Energy in finite electrical networks
a
b
a
b
Cab
E(h) =∑
a,b∈B (h(a) − h(b))2 Cab,
Compare with Douglas: E(h) =∫ 2π0
∫ 2π0 (h(η) − h(ζ))2Θ(ζ, η)dηdζ
How can we generalise this to an arbitrary domain?To an infinite graph?
Agelos Georgakopoulos
![Page 29: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/29.jpg)
Energy in finite electrical networks
a
b
a
b
Cab
E(h) =∑
a,b∈B (h(a) − h(b))2 Cab,
Compare with Douglas: E(h) =∫ 2π0
∫ 2π0 (h(η) − h(ζ))2Θ(ζ, η)dηdζ
How can we generalise this to an arbitrary domain?
To an infinite graph?
Agelos Georgakopoulos
![Page 30: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/30.jpg)
Energy in finite electrical networks
a
b
a
b
Cab
E(h) =∑
a,b∈B (h(a) − h(b))2 Cab,
Compare with Douglas: E(h) =∫ 2π0
∫ 2π0 (h(η) − h(ζ))2Θ(ζ, η)dηdζ
How can we generalise this to an arbitrary domain?To an infinite graph?
Agelos Georgakopoulos
![Page 31: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/31.jpg)
Effective conductance
We call C the effective conductance measure, because
Theorem (G & V. Kaimanovich ’12-’17+)For every locally finite network G, and every harmonicfunction h, we have
E(h) =∫∂G×∂G
(h(η) − h(ζ)
)2dC(η, ζ).
History: Douglas ’31, Naim ’57, Doob ’62, Silverstein ’74
Finite version: E(h) =∑
a,b∈B (h(a) − h(b))2 Cab
a
b
a
b
Cab
Agelos Georgakopoulos
![Page 32: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/32.jpg)
The Naim Kernel
Doob’s formula:
E(h) = q∫M2
(h(η) − h(ζ))2Θ(ζ, η)dµoηdµoζ,
for h fine-continuous quasi-everywhere [Doob ’62]. Analogousformula for graphs: [Silverstein ’74].
where the Naim Kernel Θ is defined as
Θ(ζ, η) := limzn→ζ,yn→η
G(zn, yn)G(zn,o)G(o, yn)
... in the fine topology [Naim ’57].
Agelos Georgakopoulos
![Page 33: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/33.jpg)
The Naim Kernel
Doob’s formula:
E(h) = q∫M2
(h(η) − h(ζ))2Θ(ζ, η)dµoηdµoζ,
for h fine-continuous quasi-everywhere [Doob ’62].
Analogousformula for graphs: [Silverstein ’74].
where the Naim Kernel Θ is defined as
Θ(ζ, η) := limzn→ζ,yn→η
G(zn, yn)G(zn,o)G(o, yn)
... in the fine topology [Naim ’57].
Agelos Georgakopoulos
![Page 34: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/34.jpg)
The Naim Kernel
Doob’s formula:
E(h) = q∫M2
(h(η) − h(ζ))2Θ(ζ, η)dµoηdµoζ,
for h fine-continuous quasi-everywhere [Doob ’62]. Analogousformula for graphs: [Silverstein ’74].
where the Naim Kernel Θ is defined as
Θ(ζ, η) := limzn→ζ,yn→η
G(zn, yn)G(zn,o)G(o, yn)
... in the fine topology [Naim ’57].
Agelos Georgakopoulos
![Page 35: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/35.jpg)
The Naim Kernel
Doob’s formula:
E(h) = q∫M2
(h(η) − h(ζ))2Θ(ζ, η)dµoηdµoζ,
for h fine-continuous quasi-everywhere [Doob ’62]. Analogousformula for graphs: [Silverstein ’74].
where the Naim Kernel Θ is defined as
Θ(ζ, η) := limzn→ζ,yn→η
G(zn, yn)G(zn,o)G(o, yn)
... in the fine topology [Naim ’57].
Agelos Georgakopoulos
![Page 36: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/36.jpg)
The Naim Kernel
Doob’s formula:
E(h) = q∫M2
(h(η) − h(ζ))2Θ(ζ, η)dµoηdµoζ,
for h fine-continuous quasi-everywhere [Doob ’62]. Analogousformula for graphs: [Silverstein ’74].
where the Naim Kernel Θ is defined as
Θ(ζ, η) := limzn→ζ,yn→η
G(zn, yn)G(zn,o)G(o, yn)
... in the fine topology [Naim ’57].
Agelos Georgakopoulos
![Page 37: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/37.jpg)
Convergence of the Naim Kernel
Θ(ζ, η) := limzn→ζ,yn→η
G(zn, yn)G(zn,o)G(o, yn)
Problem: Let (zi )i∈N and (yi )i∈N be independent simple randomwalks from o. Then limn,m→∞Θ(zn, ym) exists almost surely.
Agelos Georgakopoulos
![Page 38: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/38.jpg)
Convergence of the Naim Kernel
Θ(ζ, η) := limzn→ζ,yn→η
G(zn, yn)G(zn,o)G(o, yn)
Problem: Let (zi )i∈N and (yi )i∈N be independent simple randomwalks from o. Then limn,m→∞Θ(zn, ym) exists almost surely.
Agelos Georgakopoulos
![Page 39: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/39.jpg)
Random Interlacements and C
Random Interlacements I [Sznitman]:
• A Poisson point process whose ‘points’ are 2-way infinitetrajectories
• applied to study the vacanct set on the discrete 3D-torus• governed by a certain σ-finite measure ν
Theorem (G & Kaimanovich ’17+)For every transient, locally finite graph G,
C(X ,Y ) = ν(1XY W ∗).
Agelos Georgakopoulos
![Page 40: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/40.jpg)
Random Interlacements and C
Random Interlacements I [Sznitman]:• A Poisson point process whose ‘points’ are 2-way infinite
trajectories
• applied to study the vacanct set on the discrete 3D-torus• governed by a certain σ-finite measure ν
Theorem (G & Kaimanovich ’17+)For every transient, locally finite graph G,
C(X ,Y ) = ν(1XY W ∗).
Agelos Georgakopoulos
![Page 41: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/41.jpg)
Random Interlacements and C
Random Interlacements I [Sznitman]:• A Poisson point process whose ‘points’ are 2-way infinite
trajectories• applied to study the vacanct set on the discrete 3D-torus
• governed by a certain σ-finite measure ν
Theorem (G & Kaimanovich ’17+)For every transient, locally finite graph G,
C(X ,Y ) = ν(1XY W ∗).
Agelos Georgakopoulos
![Page 42: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/42.jpg)
Random Interlacements and C
Random Interlacements I [Sznitman]:• A Poisson point process whose ‘points’ are 2-way infinite
trajectories• applied to study the vacanct set on the discrete 3D-torus• governed by a certain σ-finite measure ν
Theorem (G & Kaimanovich ’17+)For every transient, locally finite graph G,
C(X ,Y ) = ν(1XY W ∗).
Agelos Georgakopoulos
![Page 43: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/43.jpg)
Random Interlacements and C
Random Interlacements I [Sznitman]:• A Poisson point process whose ‘points’ are 2-way infinite
trajectories• applied to study the vacanct set on the discrete 3D-torus• governed by a certain σ-finite measure ν
Theorem (G & Kaimanovich ’17+)For every transient, locally finite graph G,
C(X ,Y ) = ν(1XY W ∗).
Agelos Georgakopoulos
![Page 44: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/44.jpg)
Long range percolation
(Joint work in progress with J. Haslegrave.Thanks to O. Angel and G. Ray for important ideas)
Theorem (Newman & Schulman, Aizenman & Newman ’86)
In long range percolation on Z, with parameters e−λ/|x−y |s ,percolation occurs for large enough λ if s ≤ 2.
Rλn(Z2) converges (a la Benjamini-Schramm) to an instance Rλ∞of this (with s = 2) as n → ∞.
But Rλ∞(Tree) does not percolate for any λ!
How large is Rλ∞(T )?
Agelos Georgakopoulos
![Page 45: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/45.jpg)
Long range percolation
(Joint work in progress with J. Haslegrave.Thanks to O. Angel and G. Ray for important ideas)
Theorem (Newman & Schulman, Aizenman & Newman ’86)
In long range percolation on Z, with parameters e−λ/|x−y |s ,percolation occurs for large enough λ if s ≤ 2.
Rλn(Z2) converges (a la Benjamini-Schramm) to an instance Rλ∞of this (with s = 2) as n → ∞.
But Rλ∞(Tree) does not percolate for any λ!
How large is Rλ∞(T )?
Agelos Georgakopoulos
![Page 46: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/46.jpg)
Long range percolation
(Joint work in progress with J. Haslegrave.Thanks to O. Angel and G. Ray for important ideas)
Theorem (Newman & Schulman, Aizenman & Newman ’86)
In long range percolation on Z, with parameters e−λ/|x−y |s ,percolation occurs for large enough λ if s ≤ 2.
Rλn(Z2) converges (a la Benjamini-Schramm) to an instance Rλ∞of this (with s = 2) as n → ∞.
But Rλ∞(Tree) does not percolate for any λ!
How large is Rλ∞(T )?
Agelos Georgakopoulos
![Page 47: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/47.jpg)
Long range percolation
(Joint work in progress with J. Haslegrave.Thanks to O. Angel and G. Ray for important ideas)
Theorem (Newman & Schulman, Aizenman & Newman ’86)
In long range percolation on Z, with parameters e−λ/|x−y |s ,percolation occurs for large enough λ if s ≤ 2.
Rλn(Z2) converges (a la Benjamini-Schramm) to an instance Rλ∞of this (with s = 2) as n → ∞.
But Rλ∞(Tree) does not percolate for any λ!
How large is Rλ∞(T )?
Agelos Georgakopoulos
![Page 48: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/48.jpg)
Long range percolation
(Joint work in progress with J. Haslegrave.Thanks to O. Angel and G. Ray for important ideas)
Theorem (Newman & Schulman, Aizenman & Newman ’86)
In long range percolation on Z, with parameters e−λ/|x−y |s ,percolation occurs for large enough λ if s ≤ 2.
Rλn(Z2) converges (a la Benjamini-Schramm) to an instance Rλ∞of this (with s = 2) as n → ∞.
But Rλ∞(Tree) does not percolate for any λ!
How large is Rλ∞(T )?
Agelos Georgakopoulos
![Page 49: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/49.jpg)
The expected size of the TWRG
Let Cλo denote the component of a uniformly random vertex ofRλn(T ) (or Rλ∞(T )).
Theorem (G & Haslegrave, state of the art 2/17)
Aeaλ ≤ E(|Cλo |) ≤ Beebλ.
Conjecture:E(|Cλo |) ∼ λ
λ
(backed by simulations)
Agelos Georgakopoulos
![Page 50: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/50.jpg)
The expected size of the TWRG
Let Cλo denote the component of a uniformly random vertex ofRλn(T ) (or Rλ∞(T )).
Theorem (G & Haslegrave, state of the art 2/17)
Aeaλ ≤ E(|Cλo |) ≤ Beebλ.
Conjecture:E(|Cλo |) ∼ λ
λ
(backed by simulations)
Agelos Georgakopoulos
![Page 51: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/51.jpg)
Outlook
Understand TWRGsInterplay between the host group Γ and its GWRGsLet Γ act on C and see what happens
Thank you!
These slides are on-line.
Agelos Georgakopoulos
![Page 52: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/52.jpg)
Outlook
Understand TWRGs
Interplay between the host group Γ and its GWRGsLet Γ act on C and see what happens
Thank you!
These slides are on-line.
Agelos Georgakopoulos
![Page 53: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/53.jpg)
Outlook
Understand TWRGsInterplay between the host group Γ and its GWRGs
Let Γ act on C and see what happens
Thank you!
These slides are on-line.
Agelos Georgakopoulos
![Page 54: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/54.jpg)
Outlook
Understand TWRGsInterplay between the host group Γ and its GWRGsLet Γ act on C and see what happens
Thank you!
These slides are on-line.
Agelos Georgakopoulos
![Page 55: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/55.jpg)
Outlook
Understand TWRGsInterplay between the host group Γ and its GWRGsLet Γ act on C and see what happens
Thank you!
These slides are on-line.
Agelos Georgakopoulos
![Page 56: Group Walk Random Graphs - University of Bristolmb13434/pr_talks/... · Agelos Georgakopoulos Bristol, 13/2/17 Partly joint work with J. Haslegrave and with V. Kaimanovich Agelos](https://reader033.vdocuments.mx/reader033/viewer/2022042909/5f3dd35039beba1bf047cb51/html5/thumbnails/56.jpg)
Outlook
Understand TWRGsInterplay between the host group Γ and its GWRGsLet Γ act on C and see what happens
Thank you!
These slides are on-line.
Agelos Georgakopoulos