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Homework Help: Superadditive function property

  1. Feb 21, 2007 #1
    1. The problem statement, all variables and given/known data
    Show that a superadditive function has the following property:
    For any superadditive function g on XxY (cartesian product):
    f(x) = min { y' : y' = argmin g(x,y) }
    is nonincreasing in x.

    2. Relevant equations
    if g(x,y) is a superadditive on XxY, x in X, y in Y, x1 >= x2, y1 >= y2, then it satisfies the inequality:

    g(x1,y1) + g(x2,y2) >= g(x1,y2) + g(x2,y1)

    3. The attempt at a solution
    Let f(x1) = y', and suppose there is an x2 <= x1 such that f(x2) = y' then, g(x2,y1) - g(x2,y1) <= g(x2,y2) - g(x1,y2).

    I am trying to find a contradiction, so that f is increasing in x for x2.
  2. jcsd
  3. Feb 26, 2007 #2

    I think I solved this problem.
    Let x2 >= x1, and take y2 >= f(x1) = y'. Then, from the definition of f, we have:
    g(x1,f(x1)) - f(x1,y2) <= 0.

    Since g is superadditive is satisfies:
    g(x2,y2) + g(x1,f(x1)) >= g(x2,f(x1)) + g(x1,y2)

    From the first inequality above we get:
    g(x2,f(x1)) <= [g(x1,f(x1))-g(x1,y2)] + g(x2,y2) <= 0 + g(x2,y2) = g(x2,y2)

    So, we have that:
    g(x2,f(x1)) <= g(x2,y2)
    for all y2 >= f(x1). Thus, f(x2)<=f(x1) which is nonincreasing in x.
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