In a satisfiable system of linear inequalities

a11x1+···+a1nxnb1:am1x1+···+amnxnbm

we describe the inequality as forced-equal if it is satisfied with equality by every solution x = (x1,...,xn)of the system. Equivalently,Piajixibj is not forced-equal if there exists an x that satisfies the whole system and such that Piajixibj.

For example, in

x1+x22-x1-x2-2x11-x20

Short Answer

Expert verified

Describe the jthinequality is forced equal solution of x=(x1...xn)and get correct answer base on different formula to countthat satisfies the whole system and such that ƛj+iaj,i((Jx)xi1ia(j,i)xibj

Step by step solution

01

Maximum Flow Diagram of Following Network

a) The claim hold trivially of all constants are forced-equal, so assume at least one constraint is not forced-equal.

Let I be the set of constraints that are not forced equal, so that Ie I it there is some feasible solution.

we show that xI=(X1I...XnI)defined by

xI=(X1I...XnI)such Ithat is satisfied without equality.

X1=1I1χiIcharacteristic solution indeed for any fired Ijm, the constraint is satisfied by ‘x’ since summing the left – hand side and the right – hand side of the inequality over all χigives us

02

Formula base Calculation

iaj,i((Jx)xi1|I|bj

That implies,
iaj,i((Jx)xi1ia(j,i)xibj

Furthermore, if 1th constraint is not forced – equal. So that

iaj,ixiI<bj

iaj,i(Jxxi1|I|bj

So,
iaj,ixiI<bjand the constraint is satisfied without equality.

Thus, ‘x’ is a feasible solution where every
JIis satisfied without equality.

By the algorithm maintains a set of I constraints, initialized to be the empty set. And iterates.Through each constraint j=1,2,....m.

Consider thej iteration of the algorithm and let I denote the j constraint. We define a linear program, obtained from the original set of constraints as follows.

03

Conclusion

We introduct a new variable ƛi and replace the ‘j’ constraint with the following constraint :

ƛj+iaj,i(Jxxi1iaj,ixibj

We also add an objective function : max λi. we then find a optimal solution to the resulting LP.

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Most popular questions from this chapter

Question: Consider the following simple network with edge capacities as shown.

a) Show that, if the Ford-Fulkerson algorithm is run on this graph, a careless choice of updates might cause it to take 1000iterations. Imagine if the capacities were a million instead of 1000.

We will now find a strategy for choosing paths under which the algorithm is guaranteed to terminate in a reasonable number of iterations.

Consider an arbitrary directed network (G=V,E,s,t,ce)in which we want to find the maximum flow.Assume for simplicity that all edge capacities are at least 1, and define the capacity of an s - t path to be the smallest capacity of its constituent edges. The fattest path from s to t is the path with the most capacity.

b) Show that the fattest s - t path in a graph can be computed by a variant of Dijkstra’s algorithm.

c) Show that the maximum flow in Gis the sum of individual flows along at most|E|paths from s to t.

d) Now show that if we always increase flow along the fattest path in the residual graph, then the Ford-Fulkerson algorithm will terminate in at mostO(ElogF) iterations, where F is the size of the maximum flow. (Hint: It might help to recall the proof for the greedy set cover algorithm in Section 5.4.)

In fact, an even simpler rule—finding a path in the residual graph using breadth-first search— guarantees that atO(V.E)most iterations will be needed.

An edge of a flow network is called critical if decreasing the capacity of this edge results in a decrease in the maximum flow. Give an efficient algorithm that finds a critical edge in a network

Write the dual to the following linear program.

maxx+y2x+y3x+3y5x,y0

Find the optimal solutions to both primal and dual LPs

There are many common variations of the maximum flow problem. Here are four of them.

(a) There are many sources and many sinks, and we wish to maximize the total flow from all sources to all sinks.

(b) Each vertex also has a capacity on the maximum flow that can enter it.

(c) Each edge has not only a capacity, but also a lower bound on the flow it must carry.

(d) The outgoing flow from each node u is not the same as the incoming flow, but is smaller by a factor of (1-U), whererole="math" localid="1659789093525" u is a loss coefficient associated with node u.

Each of these can be solved efficiently. Show this by reducing (a) and (b) to the original max-flow problem, and reducing (c) and (d) to linear programming.

The dual of maximum flow. Consider the following network with edge capacities

(a) Write the problem of finding the maximum flow from StoTas a linear program.

(b) Write down the dual of this linear program. There should be a dual variable for each edge of the network and for each vertex other than S,T.

Now we’ll solve the same problem in full generality. Recall the linear program for a general maximum flow problem (Section 7.2).

(c) Write down the dual of this general flow LP, using a variableyefor each edge and xufor each vertexus,t.

(d) Show that any solution to the general dual LP must satisfy the following property: for any directed path from in the network, the sum of the yevalues along the path must be at least 1.

(e) What are the intuitive meanings of the dual variables? Show that anystcut in the network can be translated into a dual feasible solution whose cost is exactly the capacity of that cut.

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