and 6.855J. Network Simplex Animations

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Domenic Sullivan
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1 .8 and 6.8J Network Simplex Animations

2 Calculating A Spanning Tree Flow A tree with supplies and demands. (Assume that all other arcs have a flow of ) What is the flow in arc (,)?

3 Calculating A Spanning Tree Flow To calculate flows, iterate up the tree, and find an arc whose flow is uniquely determined. What is the flow in arc (,)?

4 Calculating A Spanning Tree Flow -6 7 What is the flow in arc (,)? 6 -

5 Calculating A Spanning Tree Flow What is the flow in arc (,6)?

6 Calculating A Spanning Tree Flow What is the flow in arc (7,)? 6

7 Calculating A Spanning Tree Flow What is the flow in arc (,6)? 7

8 Calculating A Spanning Tree Flow Note: there are two different ways of calculating the flow on (,), and both ways give a flow of. Is this a coincidence? 8

9 Calculating Simplex Multipliers for a Spanning Tree Here is a spanning tree with arc costs. How can one choose node potentials so that reduced costs of tree arcs is? Recall: the reduced cost of (i,j) is c ij - π i + π j 9

10 Calculating Simplex Multipliers for a Spanning Tree There is a redundant constraint in the minimum cost flow problem. One can set π arbitrarily. We will let π i =. What is the simplex multiplier for node?

11 Calculating Simplex Multipliers for a Spanning Tree The reduced cost of (,) is c - π + π =. Thus - + π =. What is the simplex multiplier for node 7?

12 Calculating Simplex Multipliers for a Spanning Tree The reduced cost of (,) is c 7 - π 7 + π =. Thus -6 - π + =. What is the simplex multiplier for node?

13 Calculating Simplex Multipliers for a Spanning Tree What is the simplex multiplier for node 6?

14 Calculating Simplex Multipliers for a Spanning Tree What is the simplex multiplier for node?

15 Calculating Simplex Multipliers for a Spanning Tree What is the simplex multiplier for node?

16 Calculating Simplex Multipliers for a Spanning Tree These are the simplex multipliers associated with this tree. They do not depend on arc flows, nor on costs of non-tree arcs. 6

17 Network Simplex Algorithm -, $, $, $, $, $, $, $ -, $ T L U The minimum Cost Flow Problem 7

18 Spanning tree flows - - T L U An Initial Spanning Tree Solution 8

19 Simplex Multipliers and Reduced Costs The initial simplex multipliers and reduced costs? c = T L U What arcs are violating? 9

20 Add a violating arc to the spanning tree, creating a cycle,,,,,, u, x, Arc (,) is added to the tree, T L U What is the cycle, and how much flow can be sent?

21 Send Flow Around the Cycle,,,,,, u, x, units of flow were sent along the cycle., T L U What is the next spanning tree?

22 After a pivot,,,,,, u, x, The Updated Spanning Tree, T L U In a pivot, an arc is added to T and an arc is dropped from T.

23 Updating the Multipliers The current multipliers and reduced costs T L U How can we make c π = and have other tree arcs have a reduced cost?

24 Deleting (,) from T splits T into two parts Adding to nodes on one side of the tree does not effect the reduced costs of any tree arc except (,). Why? T L U What value of should be chosen to make the reduced cost of (,) =?

25 The updated multipliers and reduced costs - - The updated multipliers and reduced costs T L U Is this tree solution optimal?

26 Add a violating arc to the spanning tree, creating a cycle,,,,, Add arc (,) to the spanning tree,,, T L U What is the cycle, and how much flow can be sent? 6

27 Send Flow Around the Cycle,,,,,, unit of flow was sent around the cycle.,, T L U What is the next spanning tree solution? 7

28 The next spanning tree solution,,,,,,,, T L U Here is the updated spanning tree solution 8

29 Updated the multipliers - - Here are the current multipliers T L U How should we modify the multipliers? 9

30 Updated the multipliers Here are the current multipliers T L U What value should be?

31 The updated multipliers Here are the updated multipliers. T L U Is the current spanning tree solution optimal?

32 The Optimal Solution -6 - Here is the optimal solution. - T L U No arc violates the optimality conditions.

33 Finding the Cycle

34 Use Depth and Predecessor depth() = ; depth() = ; replace node by pred()

35 Use Depth and Predecessor depth(9) = ; depth() = ; replace node 9 by pred(9)

36 Use Depth and Predecessor depth() = ; depth() = ; replace node by pred(); replace node by pred() 6

37 Use Depth and Predecessor depth(8) = ; depth(7) = ; replace node 8 by pred(8); replace node 7 by pred() 7

38 Use Depth and Predecessor The least common ancestor of nodes and has been found. 8

39 Updating the multipliers: use the thread and depth Suppose that arc (,8) will drop out of the tree. What is the subtree rooted at node 8? 9

40 Follow the thread starting with node What is thread(8)? 6 9

41 Follow the thread starting with node What is thread()? 6 9

42 Follow the thread starting with node What is thread()? 6 9

43 Follow the thread starting with node What is thread()? 6 9

44 Follow the thread starting with node What is thread(6)? 6 9

45 The stopping rule depth = Stopping rule: stop when depth(current node) depth(8) depth = 9

Common Mistakes. Quick sort. Only choosing one pivot per iteration. At each iteration, one pivot per sublist should be chosen.

Common Mistakes. Quick sort. Only choosing one pivot per iteration. At each iteration, one pivot per sublist should be chosen. Common Mistakes Examples of typical mistakes Correct version Quick sort Only choosing one pivot per iteration. At each iteration, one pivot per sublist should be chosen. e.g. Use a quick sort to sort the