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# 15 - NP-Completeness Design and Analysis of Algorithms...

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NP-Completeness Design and Analysis of Algorithms Andrei Bulatov

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Algorithms – NP-Completeness 15-2 NP-Completeness What are the most difficult problems in NP? A problem X is said to be NP-complete if (i) X NP (ii) for any Y NP, we have Y X Lemma If an NP-complete problem solvable in polynomial time then P = NP. Theorem (Cook, Levin) Circuit Satisfiability is NP-complete
Algorithms – NP-Completeness 15-3 Proving NP-Completeness Remark. Once we establish first "natural" NP-complete problem, others are much easier Recipe to establish NP-completeness of problem Y. Step 1. Show that Y is in NP. Step 2. Choose an NP-complete problem X. Step 3. Prove that X Y.

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Algorithms – NP-Completeness 15-4 Proving NP-Completeness Lemma If X is an NP-complete problem, and Y is a problem in NP with the property that X Y then Y is NP-complete. Proof Let W be any problem in NP. Then W X Y. By transitivity, W Y. Hence Y is NP-complete. QED
Algorithms – NP-Completeness 15-5 3-SAT: NP-Completeness Proof Suffices to show that CIRCUIT-SAT 3-SAT since 3-SAT is in NP. Let C be any circuit. Create a 3-SAT variable for each circuit element i. Theorem 3-SAT is NP-complete ¬ 0 ? ? output x 0 x 2 x 1 x 3 x 4 x 5 i x

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Algorithms – NP-Completeness 15-6 3-SAT: NP-Completeness Make circuit compute correct values at each node: add 2 clauses: add 3 clauses: add 3 clauses: Hard-coded input values and output value. add 1 clause: add 1 clause: Final step: turn clauses of length < 3 into clauses of length exactly 3. QED 3 2 x x ¬ = 5 4 1 x x x = 2 1 0 x x x = 3 2 3 2 , x x x x 5 4 1 5 1 4 1 , , x x x x x x x 2 1 0 2 0 1 0 , , x x x x x x x 0 5 = x 1 0 = x 5 x 0 x ¬ 0 ? ? output x 0 x 2 x 1 x 3 x 4 x 5
Algorithms – NP-Completeness 15-7 Hamiltonian Cycle The Hamiltonian Cycle Problem Instance: An undirected graph G = (V, E) Objective: Does there exist a simple cycle Γ that contains every node in V. YES: vertices and faces of a dodecahedron.

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Algorithms – NP-Completeness 15-8 Hamiltonian Cycle 1 3 5 1' 3' 2 4 2' 4' NO: bipartite graph with odd number of nodes .
Algorithms – NP-Completeness 15-9 Directed Hamiltonian Cycle The Directed Hamiltonian Cycle Problem Instance: A directed graph G = (V, E) Objective: Does there exist a simple directed cycle Γ that contains every node in V.

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