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Some branches of economics and game theory deal with indivisible goods – discrete items that can be traded only as a whole. For example, in combinatorial auctions there is a finite set of items, and every agent can buy a subset of the items, but an item cannot be divided between two or more agents. It is usually assumed that every agent assigns subjective utility to every subset of the items. This can be represented by one of two ways: * An ordinal utility preference relation, usually marked by . The fact that an agent prefers a set to a set is written . If the agent only weakly prefers (i.e. either prefers or is indifferent between and ) then this is written . * A cardinal utility function, usually marked by . The utility an agent gets from a set is written . Cardinal utility functions are often normalized such that , where is the empty set. A cardinal utility function implies a preference relation: implies and implies . Utility functions can have several properties. == Monotonicity == Monotonicity means that an agent always (weakly) prefers to have extra items. Formally: * For a preference relation: implies . * For a utility function: implies (i.e. ''u'' is a monotone function). Monotonicity is equivalent to the free disposal assumption: if an agent may always discard unwanted items, then extra items can never decrease the utility. 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「Utility functions on indivisible goods」の詳細全文を読む スポンサード リンク
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