ieor 4004 maximum flow problems. connectivity t t s s q1: can alice send a message to bob ? yes if...
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![Page 1: IEOR 4004 Maximum flow problems. Connectivity t t s s Q1: Can Alice send a message to Bob ? Yes if every (s,t)-cut contains at least one forward edge](https://reader037.vdocument.in/reader037/viewer/2022110304/551a933b550346e0158b5253/html5/thumbnails/1.jpg)
IEOR 4004Maximum flow problems
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Connectivity
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Q1: Can Alice senda message to Bob ?
𝐴 𝐴
Yes if every (s,t)-cut contains at least one forward edge
forward
backward
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Connectivity
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Q2: How fast?Send datain parallel
Q1: Can Alice senda message to Bob ?
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Edge capacity
ts
Idea: Data packetscan share edges(bandwidth)
1234
Two packets in parallel
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From -paths to a flow
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1234
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11
2 1
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21
1
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Conservation of flow
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v
incoming outgoing
incoming outgoing
incoming outgoing
𝑓 𝑥 (𝑠)=−4
𝑓 𝑥 (𝑡 )=4
𝑓 𝑥 (𝑖 )=∑𝑗∈𝑉𝑗𝑖∈ 𝐸
𝑥 𝑗𝑖−∑𝑗 ∈𝑉𝑖𝑗∈ 𝐸
𝑥 𝑖𝑗
net flow (excess)
valueof flow
incoming outgoing
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Feasible flow to -paths
ts
1234
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1 1
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4
111
21
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1
𝑓 𝑥 (𝑠)=−4
𝑓 𝑥 (𝑡 )=4
value of flow outgoing from
incoming outgoing
...after finite
# of stepswe reach
11
32
𝑓 𝑥 (𝑠)=−3𝑓 𝑥 (𝑠)=−2𝑓 𝑥 (𝑠)=0
𝑓 𝑥 (𝑡 )=3𝑓 𝑥 (𝑡 )=2𝑓 𝑥 (𝑡 )=0
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Forward paths do not suffice
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1 1
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22
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4
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Augmenting chain
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1234
1
1
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1 4
32
12
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1 1
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Exponentially many steps
ts
1234
MM
MM
augmenting steps
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Exponentially many steps
ts
1234
1
11
1
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22
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3
MM
MM
bad choice of augmenting chain augmenting steps
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ts
Residual network
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11
14
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and flow residual networkwith respect to flow
saturated edges (residual edges in reverse)no flow
edges (edges in both directions)
1
11 1
Forward pathAugmenting chain
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Recall: Connectivity
ts
Q1: Is there a path from s to t?
𝐴 𝐴
Yes if every (s,t)-cut contains at least one forward edge
Else No
forward
backward
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Flows and cuts
ts
1234
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1 1
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4
111
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𝑓 𝑥 (𝑡 )=4
𝐴( 𝐴)
𝐴
𝑥 ( 𝐴 )flow across a cut (forward flow – backward flow)
value of a flow
¿1+4+1+1−2−1=4
capacity of a cut (forward edges)𝑢 ( 𝐴 )=3+4+1+2=10
flow across a cut ≤ capacity of the cut
Weak duality
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Maximum flow = Minimum cut
t
1234
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4
21
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1
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s
1
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cut capacity (forward edges): flow value (forward – backward)
optimal solution
Strong duality
1
𝐴 𝐴
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Transportation problem
Factories Retail stores
Requirementfor goods
Productioncapacity
... ...
Can factories satisfy the demand of retail stores ?
ai bj
edge
if i-th factorycan deliver to j-th store
t
Maximum flow
capacity
production(capacity)
demand(capacity)
source target
necessary condition
a1
a2
an bm
b1
b2
s
Yes, if Maximum flowNo, otherwise
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Transportation problem
Factories Retail stores
Requirementfor goods
Productioncapacity
... ...
Can factories satisfy the demand of retail stores ?
t
Maximum flow
capacitylimitedproduction
(capacity)
limiteddemand(capacity)
Units of flow123
source target
necessary condition
bm
b1b2
Example 1: n=m=3a1=a2=a3=1b1=b2=b3=1
Answer: Yes!
Yes, if Maximum flowNo, otherwise
a1a2
an
s
XX
X
XX
X
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Transportation problem
Factories Retail stores
Requirementfor goods
Productioncapacity
Can factories satisfy the demand of retail stores ?
t
Maximum flow
capacity
production(capacity)
demand(capacity)
source target
necessary condition
Yes, if Maximum flowNo, otherwise
Example 2: n=m=3a1=a2=1 a3=3b1=3 b2=b3=1
Answer: No!
Maximum flow = 4 < 53rd factory does
not deliver to1st retail store
s
XX
X
XX
X
Example 1: n=m=3a1=a2=a3=1b1=b2=b3=1
𝐴
𝐴cut
of capacity 4