Region Growing for Multi-route Cuts€¦ · Multi-route Cuts and LP Formulation Region growing...
Transcript of Region Growing for Multi-route Cuts€¦ · Multi-route Cuts and LP Formulation Region growing...
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Multi-route Cuts LP Region Growing Lemma Algorithms
Region Growing for Multi-route Cuts
Siddharth Barman
Joint work with Shuchi Chawla
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Multi-route Cuts LP Region Growing Lemma Algorithms
Outline
Multi-route Cuts and LP Formulation
Region growing lemma
Algorithms
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Multi-route Cuts LP Region Growing Lemma Algorithms
Connectivity
Connectivity between two terminals is defined to be thenumber of edge disjoint paths between them.
In classical cut problems the goal is to find a set of edgeswhose removal reduces the connectivity to zero.
Relaxing to higher connectivity requirements gives usmulti-route cuts.
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Multi-route Cuts LP Region Growing Lemma Algorithms
Connectivity
Connectivity between two terminals is defined to be thenumber of edge disjoint paths between them.
In classical cut problems the goal is to find a set of edgeswhose removal reduces the connectivity to zero.
Relaxing to higher connectivity requirements gives usmulti-route cuts.
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Multi-route Cuts LP Region Growing Lemma Algorithms
Connectivity
Connectivity between two terminals is defined to be thenumber of edge disjoint paths between them.
In classical cut problems the goal is to find a set of edgeswhose removal reduces the connectivity to zero.
Relaxing to higher connectivity requirements gives usmulti-route cuts.
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Multi-route Cuts LP Region Growing Lemma Algorithms
Road Runner in Konigsberg
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Multi-route Cuts LP Region Growing Lemma Algorithms
Multi-route Cut Instance
Objective: Find a low cost set of edges such that connectivitybetween s to t to reduces to 1.
t
s
1010
10
1
11
ce = 100
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Multi-route Cuts LP Region Growing Lemma Algorithms
Multi-route Cut Instance
Objective: Find a low cost set of edges such that connectivitybetween s to t to reduces to 1.
t
s
1010
10
1
11
ce = 100
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Multi-route Cuts LP Region Growing Lemma Algorithms
Multi-route Cut Instance
Objective: Find a low cost set of edges such that connectivitybetween s to t to reduces to 1.
t
s
1010
10
1
11
ce = 100
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Multi-route Cuts LP Region Growing Lemma Algorithms
Menger’s Theorem
Menger’s Theorem: Maximum number of pairwise edgedisjoint paths between any two terminals is equal to minimumedge cut between them.
Connectivity := Number of edge disjoint paths = 2.
Menger’s Theorem: ∃ cut C with exactly 2 edges.
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Multi-route Cuts LP Region Growing Lemma Algorithms
Menger’s Theorem
Menger’s Theorem: Maximum number of pairwise edgedisjoint paths between any two terminals is equal to minimumedge cut between them.
Connectivity := Number of edge disjoint paths = 2.
Menger’s Theorem: ∃ cut C with exactly 2 edges.
s t
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Multi-route Cuts LP Region Growing Lemma Algorithms
Spectrum of Cut Problems
s
t1t2
t3
Single Source MultipleSink
Multiway Cuts
t1
t2
th
Multicuts
s1
sh
t1
th
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Multi-route Cuts LP Region Growing Lemma Algorithms
Multicut version of multi-route cut
Given: A graph G with edge costs with terminal pairs,(s1, t1), (s2, t2), · · · , (sh, th), and connectivity threshold, k.
Goal: To produce a minimum cost set of edges E ′ ⊆ E , suchthat for each i , si and ti are at most (k − 1)-edge-connectedin the graph (V ,E \ E ′).
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Multi-route Cuts LP Region Growing Lemma Algorithms
Related Work
Bruhn, Cerny, Hall and Kolman [SODA07] gave a dualitytheorem for multi-route flows and cuts on uniform capacitygraphs.
Chekuri and Khanna [ICALP08], gave a randomizedO(log2 n log h) approximation algorithm for 2-route multicuts.
Classical Multicut via region growing by Garg, Vazirani andYannakakis [STOC03]
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Multi-route Cuts LP Region Growing Lemma Algorithms
Results
EDRC: Edge Disjoint Route Cut
NDRC: Node Disjoint Route Cut
SS: Single Source Multiple Sink, MW: Multiway Cut, MC:Multicut,
Problem Previous best result Our result
SS-2-EDRC, SS-2-NDRC O(log n) O(log h)
MW-2-EDRC, MW-2-NDRC O(log n log h) O(log2 h)
MC-2-EDRC O(log2 n log h) O(log2 h)
MC-2-NDRC – O(log2 h)
SS-k-EDRC – (6,O(√
h ln h))SS-k-EDRC-Uniform – (2, 4)SS-k-EDRC (constant h) – (4, 4)
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Multi-route Cuts LP Region Growing Lemma Algorithms
Results
EDRC: Edge Disjoint Route Cut
NDRC: Node Disjoint Route Cut
SS: Single Source Multiple Sink, MW: Multiway Cut, MC:Multicut,
Problem Previous best result Our result
SS-2-EDRC, SS-2-NDRC O(log n) O(log h)
MW-2-EDRC, MW-2-NDRC O(log n log h) O(log2 h)
MC-2-EDRC O(log2 n log h) O(log2 h)
MC-2-NDRC – O(log2 h)
SS-k-EDRC – (6,O(√
h ln h))SS-k-EDRC-Uniform – (2, 4)SS-k-EDRC (constant h) – (4, 4)
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Multi-route Cuts LP Region Growing Lemma Algorithms
LP Formulation for Multicut Version
z = min∑e∈E
xe ce
s.t∑e∈E
y ie ≤ k − 1 ∀i ∈ [h]
d i (u, v) = xe + y ie ∀i ∈ [h], e = (u, v) ∈ E
d i is a metric ∀i ∈ [h]
d i (si , ti ) ≥ 1 ∀i ∈ [h]
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Multi-route Cuts LP Region Growing Lemma Algorithms
Guessing Witness Edges
We can guess k − 1 witness edges and then find min-cut.
But does not scale up for:
Higher k values, as there are(
nk−1
)possibilities
Multiple terminals
t
s
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Multi-route Cuts LP Region Growing Lemma Algorithms
Guessing Witness Edges
We can guess k − 1 witness edges and then find min-cut.
But does not scale up for:
Higher k values, as there are(
nk−1
)possibilities
Multiple terminals
t
s
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Multi-route Cuts LP Region Growing Lemma Algorithms
Guessing Witness Edges
We can guess k − 1 witness edges and then find min-cut.
But does not scale up for:
Higher k values, as there are(
nk−1
)possibilities
Multiple terminals
t
s
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Multi-route Cuts LP Region Growing Lemma Algorithms
Guessing Witness Edges
We can guess k − 1 witness edges and then find min-cut.
But does not scale up for:
Higher k values, as there are(
nk−1
)possibilities
Multiple terminals
t
s
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Multi-route Cuts LP Region Growing Lemma Algorithms
Construction Highlight: k = 2
Let cost1(C ) be the total cost of the cut minus the mostexpensive edge
Say, for s and all ti , there exists a cut C such that cost1(C )≤ α× LP contribution contained inside C .
s
t1
t2
t3
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Multi-route Cuts LP Region Growing Lemma Algorithms
Construction Highlight: k = 2
Let cost1(C ) be the total cost of the cut minus the mostexpensive edge
Say, for s and all ti , there exists a cut C such that cost1(C )≤ α× LP contribution contained inside C .
s
t1
t2
t3
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Multi-route Cuts LP Region Growing Lemma Algorithms
Construction Highlight: k = 2
Let cost1(C ) be the total cost of the cut minus the mostexpensive edge
Say, for s and all ti , there exists a cut C such that cost1(C )≤ α× LP contribution contained inside C .
s
t1
t2
t3
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Multi-route Cuts LP Region Growing Lemma Algorithms
Construction Highlight: k = 2
Let cost1(C ) be the total cost of the cut minus the mostexpensive edge
Say, for s and all ti , there exists a cut C such that cost1(C )≤ α× LP contribution contained inside C .
s
t1
t2
t3
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Multi-route Cuts LP Region Growing Lemma Algorithms
Construction Highlight: k = 2
Let cost1(C ) be the total cost of the cut minus the mostexpensive edgeSay, for s and all ti , there exists a cut C such that cost1(C )≤ α× LP contribution contained inside C .
s
t1
t2
t3
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Multi-route Cuts LP Region Growing Lemma Algorithms
Construction Highlight: k = 2
Let cost1(C ) be the total cost of the cut minus the mostexpensive edgeSay, for s and all ti , there exists a cut C such that cost1(C )≤ α× LP contribution contained inside C .
s
t1t3
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Multi-route Cuts LP Region Growing Lemma Algorithms
Construction Highlight: k = 2
Let cost1(C ) be the total cost of the cut minus the mostexpensive edge
Say, for s and all ti , there exists a cut C such that cost1(C )≤ α× LP contribution contained inside C .
s
t1
t2
t3
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Multi-route Cuts LP Region Growing Lemma Algorithms
Construction Highlight: k = 2
Total cost of the generated solution is no more than α timesthe value obtained by the linear program.
We still need to consider whether proper connectivity isachieved in the residual graph.
s
t1
t2
t3
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Multi-route Cuts LP Region Growing Lemma Algorithms
Region Growing Lemma
LP solution assigns length xe to edges
Surface area ce
Volume xece
xe
ce
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Multi-route Cuts LP Region Growing Lemma Algorithms
Region Growing Lemma: Single Edge Length
For well separated terminals s and t, there exists a cut C suchthat cost(C ) ≤ log h× LP contribution contained inside C .
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Multi-route Cuts LP Region Growing Lemma Algorithms
Region Growing Lemma: Single Edge Length
For well separated terminals s and t, there exists a cut C suchthat cost(C ) ≤ log h× LP contribution contained inside C .
s t
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Multi-route Cuts LP Region Growing Lemma Algorithms
Region Growing Lemma: Single Edge Length
For well separated terminals s and t, there exists a cut C suchthat cost(C ) ≤ log h× LP contribution contained inside C .
s t
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Multi-route Cuts LP Region Growing Lemma Algorithms
Region Growing Lemma: Single Edge Length
∃r ∈ [0, 1] such that A(r) ≤ log h × V (r).
s tA(r)r
V (r)Ball B(r)
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Multi-route Cuts LP Region Growing Lemma Algorithms
Region Growing Lemma for Multi-route Case
Even with x and y lengths there exists a cut C such that # yedges in the cut small
And x-cost(C ) ≤ O(log h)× LP contribution (x’s) containedinside C .
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Multi-route Cuts LP Region Growing Lemma Algorithms
Region Growing Lemma for Multi-route Case
Even with x and y lengths there exists a cut C such that # yedges in the cut smallAnd x-cost(C ) ≤ O(log h)× LP contribution (x’s) containedinside C .
s t
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Multi-route Cuts LP Region Growing Lemma Algorithms
Region Growing Lemma for Multi-route Case
Even with x and y lengths there exists a cut C such that # yedges in the cut smallAnd x-cost(C ) ≤ O(log h)× LP contribution (x’s) containedinside C .
s t
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Multi-route Cuts LP Region Growing Lemma Algorithms
Region Growing Lemma for Multi-route Case
∃r ∈ [0, 1] such thatAx(r) ≤ 2 log h × V x(r)y(r) < 2(k − 1)
Markov’s Inequality
s tAx(r)
y edge
r
V x(r)Ball B(r)
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Multi-route Cuts LP Region Growing Lemma Algorithms
Region Growing Lemma for 2-route
∃r ∈ [0, 1] such that
Ax(r) ≤ 2 log h × V x(r)y(r) < 2(k − 1) = 2
s tAx(r)
y edge
r
V x(r)Ball B(r)
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Multi-route Cuts LP Region Growing Lemma Algorithms
2-Route Cut Algorithms: Single Source Multiple Sink
First we find a cut S that separates terminal, say t1, from thesource via region growing lemma.Then we then remove the cut from the graph and iterate overG [V \ S ].Overall this gives a O(log h) approximation.
s
t1
t2
t3
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Multi-route Cuts LP Region Growing Lemma Algorithms
2-Route Cut Algorithms: Multicut
For 2-route multicuts in order to guarantee separation, aftercut S is found by region growing, we need to recurse on G [S ]and G [V \ S ]. This results in an O(log2 h) approximation.
s1 t1
s2 t2s4 t4
s5 t5
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Multi-route Cuts LP Region Growing Lemma Algorithms
2-Route Cut Algorithms: Multicut
For 2-route multicuts in order to guarantee separation, aftercut S is found by region growing, we need to recurse on G [S ]and G [V \ S ]. This results in an O(log2 h) approximation.
s1 t1
s2 t2s4 t4
s5 t5
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Multi-route Cuts LP Region Growing Lemma Algorithms
2-Route Cut Algorithms: Multicut
For 2-route multicuts in order to guarantee separation, aftercut S is found by region growing, we need to recurse on G [S ]and G [V \ S ]. This results in an O(log2 h) approximation.
s1 t1
s2 t2s4 t4
s5 t5
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Multi-route Cuts LP Region Growing Lemma Algorithms
k-Route Cut Algorithms
Integrality gap of the LP is Ω(k) for k + 1 route cut
s t
u
ye = kk+1
xe = 1k+1
# = k + 1# = 2k
ce = 1
ce =∞
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Multi-route Cuts LP Region Growing Lemma Algorithms
k-Route Cut Algorithms
(α, β) approximation: si and ti get α× k separated with acost of β × OPT
s t
u
ye = kk+1
xe = 1k+1
# = k + 1# = 2k
ce = 1
ce =∞
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Multi-route Cuts LP Region Growing Lemma Algorithms
k-Route Cut Algorithms
Plainly applying the lemma we get (2h, 2) and (2, 2h)Single Source Multiple Sink k-route cut using a separationidea over a strengthened LP we get a (6,O(
√h log h))
approximation.
s
t1
t2
t3
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Multi-route Cuts LP Region Growing Lemma Algorithms
Open Problems
Poly-logarithmic (bicriteria) approximation for k-route cuts fork ≥ 3.
Hardness of approximation for k-route cut problems.
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Multi-route Cuts LP Region Growing Lemma Algorithms
Open Problems
Poly-logarithmic (bicriteria) approximation for k-route cuts fork ≥ 3.
Hardness of approximation for k-route cut problems.
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Multi-route Cuts LP Region Growing Lemma Algorithms
Questions?
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