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Network Flow. Network flow. Use a graph to model material that flows through conduits. Each edge represents one conduit, and has a capacity, which is an upper bound on the flow rate = units/time. Can think of edges as pipes of different sizes.

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Network flow1
Network flow

  • Use a graph to model material that flows through conduits.

  • Each edge represents one conduit, and has a capacity, which is an upper bound on the flow rate = units/time.

  • Can think of edges as pipes of different sizes.

  • Want to compute max rate that we can ship material from a designated source to a designated sink.


Flow networks
Flow networks

  • G = (V, E) directed.

  • Each edge (u,v) has a capacity c(u, v)  0.

  • If (u, v)  E, then c (u, v) = 0.

  • Source vertex s, sink vertex t, assume s  v  t for all v  V.

  • Therefore, G is connected, and |E|  |V| - 1


Notes
Flow

  • Let G be a flow network with a capacity function c. Let s be the source and t be the sink.

  • A flow in G is a real-valued function f : V  V  R satisfying

    • Capacity constraint : For all u, v V , f(u, v) ≤ c(u, v).

    • Skew symmetry : For all u, v V, f(u, v) = - f(v, u).

    • Flow conservation : For all u V – {s, t}, ∑ v  Vf(u, v) = 0.

  • f(u, v) is called the flow from u to v, and can be positive, negative, or zero.

  • The value of a flow f is defined as | f | = ∑ v  Vf(s, v) = total flow out of source.

  • Given a flow network G with capacity function c, source s and sink t, maximum-flow problem finds a flow of maximum value.


Positive flow
Positive flow

  • The total positive flow entering a vertex is defined by

    ∑ f (u, v).

  • The total positive flow leaving a vertex is defined symmetrically.

  • The total net flow at a vertex = total positive flow leaving a vertex – total positive flow entering a vertex.

  • Flow conservation says total positive flow entering a vertex (other than s or t ) = total positive flow leaving the vertex (“flow in = flow out”)

v  V

f(u,v) > 0


Cancellation with flow
Cancellation with flow

  • If we have edges (u, v) and (v, u), skew symmetry makes it so that at most one of these edges has positive flow.

  • Say f(u, v) = 5. If we “ship” 2 units v u, we lower f (u, v) to 3. The 2 units v u cancel 2 of the u  v units.

  • Due to cancellation, a flow only gives us this “net” effect. We cannot reconstruct actual shipments from a flow.

    5 units u  v , 0 units v u is the same as 8 units u  v , 3 units v u

    The flow from u to v remain f (u, v) = 5 and f (v, u) = -5.


Networks with multiple sources and sinks
Networks with multiple sources and sinks

  • If there are multiple sources {s1, s2 , …, sm } and sinks {t1, t2 , …, tn}, we can reduce the problem of determining a maximum flow in a network with multiple sources and multiple sinks to an ordinary maximum-flow problem.

  • We add a supersources and a directed edge (s, si) with capacity c(s, si) =  for each i = 1, 2, …, m.

  • We add a supersinktand a directed edge (ti, t) with capacity c(ti, t) =  for each i = 1, 2, …, n.


Implicit summation notation
Implicit summation notation

  • Extend the notation of f, to take sets of vertices, with interpretation of summing over all vertices in the set.

  • If X and Y are sets of vertices,

    f (X, Y) = ∑∑ f (x, y).

    Therefore, can express flow conservation as f (u, V) = 0, for all u  V – {s, t}.

    For convenience, omit set braces in the implicit summations.

    In equation f (s, V - s) = f (s, V ) , V – s means V – {s}.

x  X y  Y


Lemma
Lemma

For any flow f in G = (V, E) :

  • For all X  V, f (X, X ) = 0.

  • For all X, Y  V, f (X, Y ) = - f (Y, X ).

  • For all X, Y, Z  V with X ∩ Y = ,

    f (X  Y, Z ) = f (X, Z ) + f (Y, Z ) and

    f (Z, X  Y ) = f (Z, X) + f (Z, Y )


Example
Example

Lemma : | f | = f (V, t).

Pf)


Notes
Cuts

  • A cut (S, T ) of flow network G = (V, E ) is a partition of V into S and T = V – S such that s  S and t  T.

  • For flow f, the net flow across cut (S, T ) is f (S, T ).

  • Capacity of cut (S, T ) is c (S, T ).

  • A minimum cut of G is a cut whose capacity is minimum over all cuts of G.


Lemma1
Lemma

  • Lemma (26.5) For any cut (S, T ), f (S, T ) = | f |

  • Corollary (26.6)

    The value of any flow f in a flow network G ≤ capacity of any cut.

  • Therefore, maximum flow ≤ capacity of minimum cut.

    We’ll see later that maximum flow = capacity of minimum cut.


Residual network
Residual network

  • Given a flow f in network G = (V, E ), consider a pair of vertices u, v  V. How much additional flow can we push directly from u to v ? That’s the residual capacity,

    cf (u, v) = c (u, v) – f (u, v)  0

  • Residual network: Gf (V, Ef ),

    Ef = { ( u, v )  V  V : cf (u, v) > 0}.

  • Each edge of the residual network (residual edge ) can admit a positive flow.

  • The edges in Ef is either edges in E or their reversals.

    Thus, | Ef | ≤ 2 | E |


Residual network1
Residual network

  • Residual network Gf is itself a flow network with capacities given by cf

  • Given flows f 1and f 2 , the flow sum f 1+ f 2 is defined by

    ( f 1+ f 2 ) (u, v ) = f 1 (u, v ) + f 2 (u, v ) for all u, v  V .

  • Lemma 26.2

    Given flow network G, a flow f in G, the residual network Gf , let f 'be any flow in Gf . Then the flow sum f + f 'is a flow in G with value | f + f '| = | f | + | f ' | .


Augmenting path
Augmenting path

  • Given a flow network G and a flow f, an augmenting path p is a simple path from s tot in the residual network Gf .

  • Each edge (u, v) on an augmenting path can admit some additional positive flow from u to v.

  • We call the maximum amount by which we can increase the flow on each edge in an augmenting path p, the residual capacity of p, given by cf (p) = min {cf (u, v): (u, v) is on p }.


Notes

Lemma 26.3.

Let G = (V, E ) be a flow network, let f be a flow in G, let p be an augmenting path in Gf . .Define a function fp : V  V  R by

fp (u, v) = cf (u, v)if (u, v) is on p,

- cf (u, v)if (v, u) is on p

0 otherwise.

Then fp is a flow in Gf with value | fp | = cf (p) > 0

Corollary

Given flow network G, flow f in G, and an augmenting path p in Gf , define fpas in lemma, and define f’ : V  V  R by f’ = f + fp.

Then f’ is a flow in G with value | f’ | = | f | + cf (p) > | f |.


Theorem 26 7 max flow min cut theorem
Theorem 26.7 (Max-flow min-cut theorem)

The following are equivalent :

  • f is a maximum flow.

  • f admits no augmenting path.

  • | f | = c(S,T ) for some cut (S,T ).


Ford fulkerson algorithm
Ford-Fulkerson algorithm

FORD-FULKERSON (G, s, t)

for each edge (u,v) E(G)

dof [u,v] ← f [v,u]← 0

while there is an augmenting path p in Gf

docf (p) ← min {cf (u, v): (u, v) is on p }

for each edge (u,v) in p

dof [u,v] ← f [u,v] + cf (p)

f [v,u] ← - f [u,v]


Analysis of ford fulkerson
Analysis of Ford-Fulkerson

  • Running time depends on how the augmenting path is determined. (It may not even terminate for irrational capacities!)

  • If capacities are all integers, then each augmenting path raises | f | by ≥1. If max flow is f* , then need ≤ | f* | iterations. O(E | f* | ).

  • We can improve the bound if we implement the computation of the augmenting path p with a breadth-first search ( = if the augmenting path is a shortest path from s to t in the residual network with all edge weights = 1. ) This is called Edmonds-Karp algorithm.


Analysis of edmonds karp algorithm
Analysis of Edmonds-Karp algorithm

  • Edmonds-Karp runs in O(V E2) time.

  • To prove, need to look at distances to vertices in Gf

  • Let δf (u,v) = shortest path distance u to v in Gf with unit edge weights.

  • Lemma(26.8) : For all v V – {s, t} , δf (s,v) increases monotonically with each flow augmentation.

  • Theorem(26.9) : Edmonds-Karp performs O(V E ) augmentations.

  • Use BFS to find each augmenting path in O(E) time.

  • “push-relabel algorithms” can yield better bounds (O(V2E) in Section 26.4, O(V3) in Section 26.5)


Maximum bipartite matching
Maximum bipartite matching

  • G = (V, E) (undirected ) is bipartite if we can partition V = L  R such that all edges in E go between L and R.

  • A matching is a subset of edges M  E such that for all v  V , ≤ 1 edge of M is incident on v (vertex v is matched if an edge of M is incident on it; otherwise unmatched).

  • Maximum matching : a matching of maximum cardinality.

  • Problem : given a bipartite graph G = (V, E), find a maximum matching.

  • Define flow network G’ = (V’, E’),

    • V’ = V  {s, t}.

    • E’ = {(s, u) : u  L }  { (u,v) : u  L, v  R}  { (v,t) : v  R}.

    • c(u,v)=1 for all (u,v)  E’.


Lemma2
Lemma

If M is a matching in G, then there is an integer-value flow f in G’ with value | f | = |M|. If f is an integer-valued flow in G’, then there is a matching M in G with cardinality |M| = | f | .

  • Integrality theorem : If the capacity function c takes only integral values, then the maximum flow f produced by the Ford-Fulkerson method has the property that |f | is integer-valued. Moreover, for all vertices u and v, the value of f (u,v) is an integer.


Push relabel algorithms
Push-relabel algorithms

  • Works on one vertex at a time, looking only at the vertex’s neighbors in the residual network.

  • Do not maintain flow conservation property throughout the execution.

  • Maintain a preflow f, which satisfies skew symmetry, capacity constraints, and f (V,u)≥ 0 for all vertices u V – {s}.

  • Excess flow into u is given by e(u) = f (V,u)

  • Vertex u  V – {s, t} is overflowing if e(u) > 0.


Push relabel algorithms1
Push-relabel algorithms

  • Each vertex has heights. Push flow downhill, from a higher vertex to a lower vertex.

  • The height of the source is |V |, and the height of the sink is fixed at 0.

  • All other vertex heights start at 0 and increase with time.

  • First, send as much flow as possible downhill from the source toward the sink – sends the capacity of the cut (s, V – s).

  • When the only pipes that leave a vertex u and are not already saturated with flow connect to vertices that are not downhill from u, increase its height (“relabel” u ) – increase the height to be one more than the height of the lowest of its neighbors to which it has an unsaturated pipe.

  • After all the flow that can get to the sink has arrived there, make the preflow a “legal” flow by sending the excess of overflowing vertices back to the source by relabeling vertices to above the fixed height |V| of the source.


Basic operations
Basic operations

  • Two basic operations : pushing flow excess from a vertex to one of its neighbors, and relabeling a vertex.

  • A function h : V → Nis a height function if h(s) = |V |, h(t) = 0, and h(u) ≤ h(v) + 1 for every residual edge (u, v)  Ef.

  • Lemma 26.13 : Let G=(V, E) be a flow network, let f be a preflow in G, and let h be a height function on V. For any two vertices u, v V, if h(u) > h(v) + 1, then (u,v) is not an edge in the residual graph.


Push operation
Push operation

PUSH(u,v)

Applies when : u is an overflowing , cf (u, v) > 0, and h(u) = h(v) + 1.

Action : Push df (u, v) = min(e[u], cf (u, v)) from u to v.

df (u, v) ← min(e[u], cf (u, v))

f [u, v] ← f [u, v] + df (u, v)

f [v, u] ← - f [u, v]

e[u] ← e[u] - df (u, v)

e[v] ← e[v] + df (u, v)

  • If edge (u,v) becomes saturated (cf (u, v) = 0 afterward), it is a saturating push. Otherwise, a nonsaturating push.

  • Lemma : after a nonsaturating push from u to v, v is no longer overflowing.


Relabel operation
Relabel operation

RELABEL(u)

Applies when : u is overflowing, and for all v V such that (u, v)  Ef , h(u) ≤ h(v) .

Action : increase the height of u.

h(u) ← 1 + min{h(v) : (u, v)  Ef }

  • After RELABEL(u), Ef contains at least one edge that leaves u because u is overflowing. So it gives u the greatest height allowed by the constraint on height functions,h(u) ≤ h(v) + 1 for every residual edge (u, v)  Ef.


Initialization
Initialization

INITIALIZE-PREFLOW(G, s)

for each vertex u  V [G]

do h[u]← 0

e[u]← 0

for each edge (u, v)  E[G]

do f [u, v] ← 0

f [v, u] ← 0

h[s]← | V [G] |

for each vertex u  Adj[s]

do f [s, u] ← c(s, u)

f [u, s] ← -c(s, u)

e[u] ← c(s, u)

e[s] ← e[s] - c(s, u)


Generic algorithm
Generic algorithm

GENERIC-PUSH-RELABEL(G)

INITIALIZE-PREFLOW(G,s)

while there exists an applicable push or relabel operation

do select an applicable push or relabel operation and perform it

  • Lemma : An overflowing vertex can be either pushed or relabeled.


Correctness of the push relabel method
Correctness of the push-relabel method

  • Prove that if it terminates, the preflow f is a maximum flow.

  • Prove that it terminates.

  • Lemma : Vertex heights never decrease during the execution of the algorithm. Whenever relabel operation is applied to u, h[u] increases by at least 1.

  • Lemma : During the execution of the algorithm, h is maintained as a height function.

  • Lemma : Let f be a preflow in a flow network G. Then, there is no path from s to t in the residual network Gf

  • Theorem : If the algorithm terminates, the preflow f compute is a maximum flow.


Analysis of the push relabel method
Analysis of the push-relabel method

  • Lemma : For any overflowing vertex u, there is a simple path from u to s in the residual network Gf .

  • Lemma : At any time during the execution of the algorithm,

    h[u] ≤ 2 |V | - 1 for all vertices u  V.

  • Corollary : The number of relabel operations is at most h[u] ≤ 2 |V | - 1 per vertex and at most (2 |V | - 1 )(|V | - 2 ) < 2 |V |2overall.

  • Lemma : The number of saturating pushes is < 2 |V | | E |.

  • Lemma : The number of nonsaturating pushes is <4|V |2(|V |+ |E| ).

  • Theorem : the number of basic operations is O(V 2 E)


Relable to front algorithm
Relable-to-front algorithm

  • A push-relabel algorithm but choosing the order of basic operations carefully.

  • O(V 3) running time.

  • Maintains a list of the vertices in the network

  • Beginning at the front, scan the list repeatedly selecting an overflowing vertex u and then “discharging” it. (“discharge” = push and relabel until u is no longer overflowing.)

  • Whenever a vertex is relabeled, it is moved to the front of the list and the algorithm begins its scan anew.


Admissible edges and networks
Admissible edges and networks

  • If G=(V, E) be a flow network, f is a preflow in G, and h is a height function, then (u, v) is anadmissible edgeif cf (u, v) > 0, and h(u) = h(v) + 1. Otherwise, (u, v) is inadmissible.

  • The admissible network is Gf,h =(V, Ef,h ), whereEf,his the set of admissible edges.

  • The admissible network consists of those edges through which flow can be pushed.

  • Lemma : admissible netwosk is acyclic.

  • Lemma : if a vertex u is overflowing, and (u,v) is an admissible edge, then PUSH(u,v) applies. It does not create any new admissible edges, but it may make (u,v) inadmissible.

  • Lemma : if u is overflowing, and there is no admissible edges leaving u, RELABEL(u) applies. After operation, there is at least one admissible edge leaving u, but no admissible edges entering u.


Neighbor lists
Neighbor lists

  • Neighbor list N [u] for a vertex u  V is a singly linked list of the neighbors of u in G.

  • head[N [u]] – points the first vertex in N [u]

  • next-neighbor[v] – points the next vertex in the list

    Relable-to-front algorithm cycles through each neighbor list in an arbitrary order fixed throughout the execution. For each vertex u, current[u] points to the vertex currently under consideration in N [u]

    Initially, current[u] is set to head[N [u]]


Discharging an overflowing vertex
Discharging an overflowing vertex

  • An overflowing vertex u is discharged by pushing all its excess flow through admissible edges, relabeling u as necessary to cause edges leaving u to become admissible.

    DISCHARGE(u)

    while e[u] > 0

    do v ← current[u]

    if v = NIL

    then RELABEL(u)

    current[u] ← head[N[u]]

    elsif cf (u, v) > 0, and h(u) = h(v) + 1

    then PUSH(u,v)

    else current[u] ← next-neighbor[v]


Relable to front algorithm1
Relable-to-front algorithm

RELABLE-TO-FRONT(G,s,t)

INITIALIZE-PREFLOW(G,s)

L ← V[G] – {s, t}, in any order

for each vertex u V[G] – {s,t}

do current[u] ← head[N[u]]

u ← head[L]

while u  NIL

do old-height ← h[u]

DISCHARGE(u)

if h[u] > old-height

then move u to the front of list L

u ← next[u]


Correctness of relabel to front
Correctness of relabel-to-front

  • An implementation of generic push-relabel algorithm – performs push and relabel operations only when they apply.

  • Need to show when relabel-to-front terminates, no basic operations apply.

  • Loop invariant : List L is a topological sort of the vertices in the admissible network and no vertex before u in the list has excess flow.