Approximation Algorithms
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1 Approximation Algorithms
2 Announcements Problem Set 9 due right now. Final exam this Monday, Hewlett 200 from 12:15PM- 3:15PM Please let us know immediately after lecture if you want to take the final at an alternate time and haven't yet told us. Final exam review sessions: Saturday from 3:00PM 5:00PM in Gates 104 Sunday from 1:00PM 3:00PM in Gates 104 Friday Four Square today, 4:15, outside of Gates.
3 Decision vs. Function Problems All of the problems we have encountered this quarter are decision problems with yes/no answers. Many interesting questions do not have yes/no answers: What is 1 + 1? How many steps does it take this TM to halt on this string? What is the shortest path from u to v? These questions are called function problems and require some object to be found.
4 NP-Hard Function Problems Some function problems are NP-hard: there is a polynomial-time reduction from any problem in NP to those problems. Examples: What is the largest independent set in a graph? What is the length of the longest path in a graph? What is the minimum number of colors required to color a graph? A polynomial-time solver for any of these could be used to build a polynomial-time decider for any problem in NP.
5 NP-Hard Function Problems The maximum independent set problem (MAX-INDSET) is Given a graph G, find an independent set S in G of the largest possible size. MAX-INDSET is NP-hard by a reduction from independent set: M = On input G, k : Find a maximum independent set in G, call it S. If S k, accept; otherwise, reject.
6 NP-Hard Function Problems Because they can be used to solve any problem in NP, NP-hard function problems are believed to be computationally infeasible. If any NP-hard function problem has a polynomialtime solution, then P = NP. Since the P = NP question is still open, no NP-hard function problems are known to have polynomial-time solutions.
7 Approximation Algorithms An approximation algorithm is an algorithm for yielding a solution that is close to the correct solution to a problem. The definition of close depends on the problem: Maximum independent set: Find an independent set that is not much smaller than the maximum independent set. Longest path: Find a long path that is not much smaller than the longest path. Graph coloring: Find a coloring of the graph that does not use many more colors than the optimal coloring.
8 How Good is an Approximation? Approximation algorithms are only useful if there is some connection between the approximate answer and the real answer. We say that an approximation algorithm is a k-approximation if its answer is always within a factor of k of the optimal solution. A 2-approximation to the graph coloring problem always finds a coloring that uses at most twice the optimal number of colors. A 2/3-approximation to the longest path problem always finds a path that is at least 2/3 as long as the optimal path.
9 Why Approximations Matter A classic NP-hard problem is job scheduling. Given a set of tasks and a set of workers to perform the tasks, how do you distribute tasks to workers to minimize the total time required (assuming the workers work in parallel?) Applications to manufacturing, operating systems, etc. Although finding an optimal solution is NP-hard, there are polynomial-time algorithms for finding 4/3-approximate solutions. If we just need a good enough answer, the NPhardness of job scheduling is not a deterrent to its use.
10 A Hamiltonian cycle in an undirected graph G is a simple cycle that visits every node in G.
11 A Hamiltonian cycle in an undirected graph G is a simple cycle that visits every node in G.
12 A Hamiltonian cycle in an undirected graph G is a simple cycle that visits every node in G.
13 A Hamiltonian cycle in an undirected graph G is a simple cycle that visits every node in G.
14 A Hamiltonian cycle in an undirected graph G is a simple cycle that visits every node in G.
15 Hamiltonian Cycles The undirected Hamiltonian cycle problem is Given an undirected graph G, does G contain a Hamiltonian cycle? As a formal language: UHAMCYCLE = { G G is an undirected graph containing a Hamiltonian cycle } Important fact: UHAMCYCLE is NP-complete. Reduction from UHAMPATH.
16 Given a complete, undirected, weighted graph G, the traveling salesman problem (TSP) is to find a Hamiltonian cycle in G of least total cost.
17 Given a complete, undirected, weighted graph G, the traveling salesman problem (TSP) is to find a Hamiltonian cycle in G of least total cost.
18 Given a complete, undirected, weighted graph G, the traveling salesman problem (TSP) is to find a Hamiltonian cycle in G of least total cost.
19 Cost: Given a complete, undirected, weighted graph G, the traveling salesman problem (TSP) is to find a Hamiltonian cycle in G of least total cost.
20 Given a complete, undirected, weighted graph G, the traveling salesman problem (TSP) is to find a Hamiltonian cycle in G of least total cost.
21 Cost: Given a complete, undirected, weighted graph G, the traveling salesman problem (TSP) is to find a Hamiltonian cycle in G of least total cost.
22 Given a complete, undirected, weighted graph G, the traveling salesman problem (TSP) is to find a Hamiltonian cycle in G of least total cost.
23 Cost: Given a complete, undirected, weighted graph G, the traveling salesman problem (TSP) is to find a Hamiltonian cycle in G of least total cost.
24 Cost: 39 (This is the optimal solution) 15 Given a complete, undirected, weighted graph G, the traveling salesman problem (TSP) is to find a Hamiltonian cycle in G of least total cost.
25 TSP, Formally Given as input A complete, undirected graph G, and a set of edge weights, which are positive integers, the TSP is to find a Hamiltonian cycle in G with least total weight. Note that since G is complete, there has to be at least one Hamiltonian cycle. The challenge is finding the least-cost cycle.
26 TSP There are many variations on TSP: Directed versus undirected graphs. Complete versus incomplete graphs. Finding a path versus finding a cycle. Nonnegative weights versus arbitrary weights. All of these problems are known to be NP-hard. The best-known algorithms have horrible runtimes: O(n 2 2 n ).
27 TSP is NP-Hard To show that TSP is NP-hard, we reduce the undirected Hamiltonian cycle problem to TSP.
28 TSP is NP-Hard To show that TSP is NP-hard, we reduce the undirected Hamiltonian cycle problem to TSP.
29 TSP is NP-Hard To show that TSP is NP-hard, we reduce the undirected Hamiltonian cycle problem to TSP.
30 TSP is NP-Hard To show that TSP is NP-hard, we reduce the undirected Hamiltonian cycle problem to TSP.
31 TSP is NP-Hard To show that TSP is NP-hard, we reduce the undirected Hamiltonian cycle problem to TSP. Cost 1 Cost 2
32 TSP is NP-Hard To show that TSP is NP-hard, we reduce the undirected Hamiltonian cycle problem to TSP. Cost 1 Cost 2 If there is a Hamiltonian cycle in the original graph, there is a TSP solution of cost n in the new graph (where n is the number of nodes) If there is no Hamiltonian path, the TSP solution has cost at least n.
33 TSP is NP-Hard To show that TSP is NP-hard, we reduce the undirected Hamiltonian cycle problem to TSP. Cost 1 Cost 2 If there is a Hamiltonian cycle in the original graph, there is a TSP solution of cost n in the new graph (where n is the number of nodes) If there is no Hamiltonian cycle, the TSP solution has cost at least n + 1.
34 Answering TSP TSP has great practical importance, and we want to be able to answer it. Determining the shortest subway routes to link stations in a ring. Determining the fastest way to move a mechanical drill so it can drill in the appropriate locations. Because TSP is NP-hard, obtaining an exact answer is impractically hard. Can we approximate it?
35 Approximating TSP An approximation algorithm to TSP is a k-approximation if it finds a Hamiltonian cycle whose cost is at most k times the optimal solution. If the optimal solution has cost 10, a 2-approximation would return a Hamiltonian cycle of cost between 10 and 20, inclusive. If the optimal solution has cost 10, a 3-approximation would return a Hamiltonian cycle of cost between 10 and 30, inclusive. For what values of k is there an efficient k-approximation to TSP?
36 Theorem: If P NP, then there is no polynomial-time k-approximation to TSP for any natural number k.
37 Hardness of Approximation The proof that TSP is hard to approximate is based on a beautiful construction: If we could obtain a k-approximation to TSP in polynomial-time, we would have a polynomial-time algorithm for UHAMCYCLE. Since UHAMCYCLE is NP-complete, this is a contradiction if P NP.
38 The Construction Proof Sketch: Suppose, for the sake of contradiction, that there is a k-approximation to TSP for some k.
39 The Construction Proof Sketch: Suppose, for the sake of contradiction, that there is a k-approximation to TSP for some k.
40 The Construction Proof Sketch: Suppose, for the sake of contradiction, that there is a k-approximation to TSP for some k.
41 The Construction Proof Sketch: Suppose, for the sake of contradiction, that there is a k-approximation to TSP for some k. Cost 1 Cost??
42 The Construction Proof Sketch: Suppose, for the sake of contradiction, that there is a k-approximation to TSP for some k. If the original graph of n nodes has a Hamiltonian cycle, the TSP solution has cost n. The k-approximation algorithm thus must hand back a Hamiltonian cycle of cost at most kn. Cost 1 Cost?? What if we made the cost of the red edges so large that any solution using them must cost more than this?
43 The Construction Proof Sketch: Suppose, for the sake of contradiction, that there is a k-approximation to TSP for some k. If the original graph of n nodes has a Hamiltonian cycle, the TSP solution has cost n. The k-approximation algorithm thus must hand back a Hamiltonian cycle of cost at most kn. Cost 1 Cost?? What if we made the cost of the red edges so large that any solution using them must cost more than this?
44 The Construction Proof Sketch: Suppose, for the sake of contradiction, that there is a k-approximation to TSP for some k. If the original graph of n nodes has a Hamiltonian cycle, the TSP solution has cost n. The k-approximation algorithm thus must hand back a Hamiltonian cycle of cost at most kn. Cost 1 Cost?? What if we made the cost of the red edges so large that any solution using them must cost more than this?
45 The Construction Proof Sketch: Suppose, for the sake of contradiction, that there is a k-approximation to TSP for some k. If the original graph of n nodes has a Hamiltonian cycle, the TSP solution has cost n. The k-approximation algorithm thus must hand back a Hamiltonian cycle of cost at most kn. Cost 1 Cost kn + 1 What if we made the cost of the red edges so large that any solution using them must cost more than this?
46 The Construction Proof Sketch: Suppose, for the sake of contradiction, that there is a k-approximation to TSP for some k. Cost 1 Cost kn + 1
47 The Construction Proof Sketch: Suppose, for the sake of contradiction, that there is a k-approximation to TSP for some k. If the k-approximation hands back a solution of cost at most kn, the original graph has a Hamiltonian cycle. Cost 1 Cost kn + 1 If the k-approximation hands back a solution of cost at least kn + 1, the original graph has no Hamiltonian cycles.
48 The Construction Proof Sketch: Suppose, for the sake of contradiction, that there is a k-approximation to TSP for some k. If the k-approximation hands back a solution of cost at most kn, the original graph has a Hamiltonian cycle. Cost 1 Cost kn + 1 If the k-approximation hands back a solution of cost at least kn + 1, the original graph has no Hamiltonian cycles.
49 What Just Happened?
50 Cost of optimal TSP solution What Just Happened?
51 What Just Happened? Cost of optimal TSP solution Cost of a yes answer.
52 What Just Happened? Cost of the best possible no answer. Cost of optimal TSP solution Cost of a yes answer.
53 What Just Happened? Cost of the best possible no answer. Cost of optimal TSP solution k times the cost of a yes answer. Cost of a yes answer.
54 What Just Happened? Cost of the best possible no answer. Cost of optimal TSP solution k times the cost of a yes answer. Cost of a yes answer.
55 What Just Happened? Cost of the best possible no answer. Cost of optimal TSP solution k times the cost of a yes answer. Cost of a yes answer.
56 What Just Happened? Create an enormous gap between the TSP answer for yes and no instances of the Hamiltonian cycle problem. Make the gap so large that the worst possible k-approximate answer to a yes instance is distinguishable from the best possible no instance. Decide whether there is a Hamiltonian cycle by measuring this difference.
57 The PCP Theorem
58 Approximating 3SAT 3SAT is one of the canonical NP-complete problems. What would it mean to approximate a 3SAT answer?
59 Approximating 3SAT The MAX-3SAT problem is Given a 3CNF formula φ, find a satisfying assignment the maximizes the number of satisfied clauses. Idea: If we can satisfy the entire formula, then MAX- 3SAT finds a satisfying assignment. If not, MAX-3SAT finds the best assignment that it can. There is a known randomized 7/8-approximation algorithm for MAX-3SAT. Is it possible to do better?
60 The 3-CNF Value If φ is a 3-CNF formula, the value of φ (denoted val(φ)) is the fraction of the clauses of φ that can be simultaneously satisfied by any assignment. If φ is satisfiable, val(φ) = 1. All of the clauses are satisfiable. If φ is unsatisfiable, val(φ) < 1. Some (but not all) clauses can be satisfied.
61 The PCP Theorem For any language L NP, There exists a poly-time reduction f from L to MAX-3SAT so For any string w: If w L, then val(f( w)) = 1. If w L, then val(f( w)) 7/8.
62 The PCP Theorem For any language L NP, There exists a poly-time reduction f from L to MAX-3SAT so For any string w: If w L, then val(f( w)) = 1. If the original answer is yes, the 3-CNF formula is satisfiable. If w L, then val(f( w)) 7/8.
63 The PCP Theorem For any language L NP, There exists a poly-time reduction f from L to MAX-3SAT so For any string w: If w L, then val(f( w)) = 1. If the original answer is yes, the 3-CNF formula is satisfiable. If w L, then val(f( w)) 7/8. If the original answer is no, at most 7/8 of the clauses can be satisfied.
64 What Does This Mean? Our proof that (unless P = NP) TSP cannot be efficiently approximated works by building up a gap between yes answers and no answers. The PCP theorem states that any problem in NP can be reduced to MAX-3SAT such that All of the clauses are satisfiable if the original answer is yes. At most 7/8 of the clauses are satisfiable if the answer is no. This gap of 1/8 of the clauses is large enough to preclude approximations beyond the known 7/8-approximation.
65 Theorem: If P NP, then for any r > 7/8, there is no polynomial-time r-approximation to MAX-3SAT.
66 MAX-3SAT is Inapproximable Theorem: If P NP, then for any r > 7/8, there is no polynomial-time r-approximation to MAX-3SAT.
67 MAX-3SAT is Inapproximable Theorem: If P NP, then for any r > 7/8, there is no polynomial-time r-approximation to MAX-3SAT. Fraction of Satisfiable Clauses
68 MAX-3SAT is Inapproximable Fraction satisfiable in a yes answer Theorem: If P NP, then for any r > 7/8, there is no polynomial-time r-approximation to MAX-3SAT. Fraction of Satisfiable Clauses
69 MAX-3SAT is Inapproximable Fraction satisfiable in a yes answer 7/8 Fraction satisfiable in a no answer Theorem: If P NP, then for any r > 7/8, there is no polynomial-time r-approximation to MAX-3SAT. Fraction of Satisfiable Clauses
70 MAX-3SAT is Inapproximable Fraction satisfiable in a yes answer r times the fraction satisfiable in a yes answer 7/8 Fraction satisfiable in a no answer Theorem: If P NP, then for any r > 7/8, there is no polynomial-time r-approximation to MAX-3SAT. Fraction of Satisfiable Clauses
71 MAX-3SAT is Inapproximable Fraction satisfiable in a yes answer r times the fraction satisfiable in a yes answer 7/8 Fraction satisfiable in a no answer Theorem: If P NP, then for any r > 7/8, there is no polynomial-time r-approximation to MAX-3SAT. Fraction of Satisfiable Clauses
72 Effects on Approximability Assuming P NP, there is a limit to how well we can approximate 3SAT. Look at our reduction from 3SAT to INDSET: ( x y z ) ( x y z ) ( x y z )
73 Effects on Approximability Assuming P NP, there is a limit to how well we can approximate 3SAT. Look at our reduction from 3SAT to INDSET: ( x y z ) ( x y z ) ( x y z )
74 Effects on Approximability Assuming P NP, there is a limit to how well we can approximate 3SAT. Look at our reduction from 3SAT to INDSET: ( x y z ) ( x y z ) ( x y z ) y y y x z x z x z
75 Effects on Approximability Many reductions preserve the size of some difficult-toapproximate quantity. Assuming if P NP: Because MAX-3SAT is not efficiently 7/8-approximable, MAX-INDSET is not efficiently 7/8-approximable either. Because MAX-INDSET is not efficiently 7/8-approximable, MAX-CLIQUE is not efficiently 7/8-approximable. Because MAX-INDSET is not efficiently ρ-approximable, MAX-SETPACK is not efficiently 7/8-approximable. Not all reductions have this property; some NP-hard problems can be efficiently approximated to very high precision.
76 Oh, and a proof of the PCP theorem?
77 Oh, and a proof of the PCP theorem?
78 Summary of Approximability While many NP-hard problems can be efficiently approximated, certain NP-hard problems cannot unless P = NP. Efficiently approximating some NP-hard problems too well would solve NP-complete problems. The PCP theorem states that the gap between yes and no answers can be so large that approximating the solution would essentially solve the problem.
79 Where to go from here?
80 Systems Theory
81 Systems Theory CS154 Intro to Automata and Complexity Theory An in-depth treatment of automata, computability, and complexity. Emphasis on theoretical results in automata theory and complexity. Launching point for more advanced courses (CS254, CS354)
82 Systems Theory CS258 Intro to Programming Language Theory Explore questions of computability in terms of recursion and recursive functions. Excellent complement to the material in this course; highly recommended. Offered every other year but available next quarter; consider checking it out!
83 Systems Theory CS109 Intro to Probability for Computer Scientists Learn to embrace randomness. Use your newly acquired proof skills in an entirely different domain. Learn why Mehran Sahami is so awesome.
84 Systems Theory CS255 Intro to Cryptography How can you use hard problems to your advantage? How can NP-completeness help you keep secrets?
85 Systems Theory CS161 Design and Analysis of Algorithms Learn how to approach new problems and solve them efficiently. Learn how to deal with NP-completeness. Learn how to ace job interviews
86 Systems Theory CS143 Compilers Watch automata, grammars, undecidability, and NP-completeness come to life by building a complete working compiler from scratch. See just how much firepower you can get from all this material.
87 Systems Theory CS107 Computer Organization and Systems You don't need to be a theoretician to love computer science! If you want to learn how the machine works under the hood, look no further.
88
89 There are more problems to solve than there are programs to solve them.
90 Where We've Been Given this hard theoretical limit, what can we compute? How powerful of a computer do we need to reach this limit? Of what we can compute, what can we compute efficiently? What techniques from mathematics can we use to reason about this?
91 What We've Covered Sets Graphs Proof Techniques Relations Induction Logic Pigeonhole Principle Trees DFAs NFAs Regular Expressions CFGs PDAs Pumping Lemmas Turing Machines Undecidability Unrecognizability Reductions Rice's Theorem Time Complexity P NP
92 Final Thoughts
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