Supermodular function: Difference between revisions

Revision as of 18:14, 15 December 2013

Revision as of 06:32, 30 March 2015

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Supermodularity and submodularity are also defined for functions defined over subsets of a larger set. Intuitively, a submodular function over the subsets demonstrates "diminishing returns". There are specialized techniques for optimizing submodular functions.

Let ''S'' be a finite set. A function <math>f\colon 2^S \to R</math> is submodularsupermodular if for any <math>A \subset B \subset S</math> and <math>x \in S \setminus B</math>, <math>f(A \cup \{x\})-f(A) \geqleq f(B \cup \{x\})-f(B)</math>. For supermodularitysubmodularity, the inequality is reversed.

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A simple illustrative example motivates this definition of submodular. Let S be a set of different foods, <math>M \subset S</math> a meal, and <math>f(M)</math> the "goodness" of that meal. Then A above is one meal, and B is A but with even more options. Let x be ice cream. Adding ice cream to a meal is always good, but it is best if there is not already a dessert. If A and B either both have a dessert or both do not, then adding ice cream to them is comparably good. But if A does not have dessert and B does, then the effect of adding ice cream is more pronounced in A.

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The definition of submodularitysupermodularity can equivalently be formulated as

:<math> f(A)+f(B) \geqleq f(A \cap B) + f(A \cup B) </math>

for all subsets ''A'' and ''B'' of ''S''.