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CSC 373H / L0101        Lecture Summary for Week 10             Winter 2005
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Network flow examples:

  - Project selection (continued):  See section 7.10 in textbook.

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Linear Programming
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"linear program" = optimization problem that consists of:

  - "objective function": c_1 x_1 + c_2 x_2 + ... + c_n x_n
    c_i constants, x_i variables (all over real numbers)

  - "constraints": linear, i.e., of the form
        a_{i,1} x_1 + a_{i,2} x_2 + ... + a_{i,n} x_n  <= / = / >=  b_i
    for i = 1, 2, ..., m.

  - problem: find real values of x_i's that maximize/minimize objective
    function and satisfy all constraints.

  - "Integer programming": all constants and variables are integers.
    NP-complete.

Examples:

  - "Political problem" example from "Introduction to Algorithms, 2nd ed."
    by Cormen et al., pp. 770-772.

  - Shortest paths:  Given graph G=(V,E) with weights w(e) for all e in E,
    construct linear program with variables d_v for each v in V:
        maximize:    d_t
        subject to:  d_v <= d_u + w(u,v)  for each (u,v) in E
                     d_s = 0
    Minimizing d_t doesn't work because it allows settings of d_v smaller
    than true distances (e.g., d_v = 0 for all v in V).  Maximizing works
    because constraints force d_v to be no more than shortest distance and
    maximization forces d_v to be at least shortest distance, for all v.

  - Network flows:  Given network N=(V,E) with capacities c(e) for e in E,
    construct linear program with variables f_e for each e in E:
        maximize:    SUM_{e in E+(s)} f_e
        subject to:
            f_e <= c(e)  for all e in E
            SUM_{e in E-(v)} f_e = SUM_{e in E+(v)} f_e  for all v in V
    where E-(v) = in-edges of vertex v and E+(v) = out-edges of vertex v,
    for all v in V.  This is a direct re-statement of network flow problem.

  - Vertex Cover:
      . Problem definition:
        Input:  Undirected graph G=(V,E).
        Output:  Subset of vertices C that "covers" every edge
            (i.e., each edge has at least one endpoint in C),
            with minimum size.
      . Representing as integer program:
        use variable x_i for each vertex v_i in V
            minimize:    x_1 + x_2 + ... + x_n
            subject to:  x_i + x_j >= 1  for all (v_i,v_j) in E
                         x_i in {0,1}  for all v_i in V
        Integer program is completely equivalent to original problem,
        including NP-completeness (so no polytime algorithm).
      . Linear relaxation:
        remove restriction of x_i to integer values
            minimize:    x_1 + x_2 + ... + x_n
            subject to:  x_i + x_j >= 1  for all (v_i,v_j) in E
                         0 <= x_i <= 1  for all v_i in V
        Solution can be found in polytime, but may include fractional
        values of x_i's.
        Example: G=(V,E) where V={1,2,3}, E={(1,2),(2,3),(1,3)} becomes
            minimize:    x_1 + x_2 + x_3
            subject to:  x_1 + x_2 >= 1
                         x_2 + x_3 >= 1
                         x_1 + x_3 >= 1
                         0 <= x_1, x_2, x_3 <= 1
        with solution x_1 = x_2 = x_3 = 1/2.
      . Rounding:
        Compute optimal solution to linear program: x'_1, x'_2, ..., x'_n.
        Create cover as follows:
            for each v_i in V, put v_i in C iff x'_i >= 1/2.
        C is a cover because constraint x_i + x_j >= 1 guarantees at least
        one of x'_i, x'_j >= 1/2 for each edge (v_i,v_j).
      . 2-approximation bound:
        Consider minimum vertex cover C*.  For i = 1,...,n, let x*_i = 1 if
        v_i in C*; x*_i = 0 otherwise.  x*_1, ..., x*_n is a solution to
        linear program that satisfies all constraints with 0-1 values so
                        SUM x'_i <= SUM x*_i = |C*|
        (since x'_i is optimal solution to linear program with no
        restriction on values).
        For i = 1,...,n, let x~_i = 1 if x'_i >= 1/2; x~_i = 0 othewise.
        Then, for each i, x~_i <= 2 x'_i so
            |C| = SUM x~_i <= 2 SUM x'_i <= 2 |C*| (by equation above)
        Hence, |C| is no more than twice the size of a minimum vertex
        cover.

  - Weighted vertex cover: section 11.6 (simple generalization).