# Lec2 - CS151 Complexity Theory Lecture 2 Reductions reductions are the main tool for relating problems to each other given two languages L1 and L2

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CS151 Complexity Theory Lecture 2 March 31, 2011

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March 31, 2011 2 Reductions reductions are the main tool for relating problems to each other • given two languages L 1 and L 2 we say “ L 1 reduces to L 2 ” and we write “ L 1 ≤ L 2 ” to mean: there exists an efficient (for now, poly-time) algorithm that computes a function f s.t. • x L 1 implies f(x) L 2 • x L 1 implies f(x) L 2
March 31, 2011 3 Reductions positive use: given new problem L 1 reduce it to L 2 that we know to be in P . Conclude L 1 in P (how?) e.g. bipartite matching max flow – formalizes “ L 1 as easy as L 2 yes no yes no L 1 L 2 f f

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March 31, 2011 4 Reductions negative use : given new problem L 2 reduce L 1 (that we believe not to be in P ) to it. Conclude L 2 not in P if L 1 not in P (how?) e.g. satisfiability graph 3-coloring – formalizes “ L 2 as hard as L 1 yes no yes no L 1 L 2 f f
March 31, 2011 5 Reductions Example reduction: 3SAT = { φ : φ is a 3-CNF Boolean formula that has a satisfying assignment } (3-CNF = AND of OR of 3 literals) IS = { (G, k) | G is a graph with an independent set V’ µ V of size ≥ k } (ind. set = set of vertices no two of which are connected by an edge)

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March 31, 2011 6 Ind. Set is NP-complete The reduction f: given φ = (x y ¬ z) ( ¬ x w z) (…) we produce graph G φ : x y ¬ z ¬ x w z one triangle for each of m clauses edge between every pair of contradictory literals set k = m
March 31, 2011 7 Reductions φ = (x y ¬ z) ( ¬ x w z) (…) Claim: φ has a satisfying assignment if and only if G has an independent set of size at least k proof? Conclude that 3SAT ≤ IS. x y ¬ z ¬ x w z

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March 31, 2011 8 Completeness complexity class C language L is C-complete if L is in C every language in C reduces to L very important concept formalizes “L is hardest problem in complexity class C
March 31, 2011 9 Completeness Completeness allows us to reason about the entire class by thinking about a single concrete problem related concept: language L is C-hard if every language in C reduces to L

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March 31, 2011 10 Completeness May ask: how to show every language in C reduces to L? in practice, shown by reducing known C - complete problem to L often not hard to find “1 st ” C-complete language, but it might not be “natural”
March 31, 2011 11 Completeness Example: NP = the set of languages L where L = { x : 9 y, |y| |x| k , (x, y) R } and R is a language in P. one NP -complete language “bounded halting”: BH = { (M, x, 1 m ) : 9 y, |y| |x| k s.t. M accepts (x, y) in at most m steps } challenge is to find natural complete problem Cook 71 : SAT NP -complete

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March 31, 2011 12 Summary problems function, decision language = set of strings complexity class = set of languages
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Lec2 - CS151 Complexity Theory Lecture 2 Reductions reductions are the main tool for relating problems to each other given two languages L1 and L2

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