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Approximate Algorithms (chap. 35). Motivation: Many problems are NP-complete, so unlikely find efficient algorithms Three ways to get around: If input size is small, exponential algorithm is OK. Isolate important special case and find poly algorithms for them.

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Approximate algorithms chap 35
Approximate Algorithms (chap. 35)

  • Motivation:

    • Many problems are NP-complete, so unlikely find efficient algorithms

    • Three ways to get around:

      • If input size is small, exponential algorithm is OK.

      • Isolate important special case and find poly algorithms for them.

      • Find near-optimal solutions in poly time.

  • So approximate algorithms:

    • An algorithm returning a near-optimal solution is called approximate algorithm.


Vertex cover problem
Vertex-cover problem

  • Vertex cover: given an undirected graph G=(V,E), then a subset V'V such that if (u,v)E, then uV' or v V' (or both).

  • Size of a vertex cover: the number of vertices in it.

  • Vertex-cover problem: find a vertex-cover of minimal size.


Vertex cover problem1
Vertex-cover problem

  • Vertex-cover problem is NP-complete. (See section 34.5.2).

    • Vertex-cover belongs to NP.

    • Vertex-cover is NP-hard (CLIQUEPvertex-cover.)

      • Reduce <G,k> where G=<V,E> of a CLIQUE instance to <G',|V|-k> where G'=<V,E'> where E'={(u,v): u,vV, uv and <u,v>E} of a vertex-cover instance.

  • So find an approximate algorithm.


Approximate ratio
Approximate Ratio

  • C* is the cost of optimal solution and C is the cost of an approximate algorithm

  • (n)=max(C/C*, C*/C) where n is size of problem input

  • If (n)=1, then the algorithm is an optimal algorithm

  • The larger (n), the worse the algorithm




2 approximate vertex cover
2-approximate vertex-cover

  • Theorem 35.1 (page 1026).

    • APPROXIMATE-VERTEX-COVER is a poly time 2-approximate algorithm, i.e., the size of returned vertex cover set is at most twice of the size of optimal vertex-cover.

  • Proof:

    • It runs in poly time

    • The returned C is a vertex-cover.

    • Let A be the set of edges picked in line 4 and C* be the optimal vertex-cover.

      • Then C* must include at least one end of each edge in A and no two edges in A are covered by the same vertex in C*, so |C*||A|.

      • Moreover, |C|=2|A|, so |C|2|C*|.


Another kind of approximate algorithm
Another kind of approximate algorithm

  • Approximate string matching (also called string matching allowing errors):

    • Find all the substrings in text T that are close to pattern P.

    • Edit distance: P is said to be of distance k to a string Q if P can be transformed to Q with k following character operations: insertion, deletion, and substitution.

    • May have other operations and different operations have different costs.

    • Refer to the handout paper by Sun Wu and Udi Manber.


Algorithms
Algorithms

  • Sequential

  • Parallel

  • Approximate

  • deterministic

  • Random

  • Probabilistic

  • Genetic

    • Evolution and optimization


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