In the mathematical fields of general topology and descriptive set theory, a meagre set (also called a meager set or a set of first category) is a set that, considered as a subset of a (usually larger) topological space, is in a precise sense small or negligible. The meagre subsets of a fixed space form a σ-ideal of subsets; that is, any subset of a meagre set is meagre, and the union of countably many meagre sets is meagre.
General topologists use the term Baire space to refer to a broad class of topological spaces on which the notion of meagre set is not trivial (in particular, the entire space is not meagre). Descriptive set theorists mostly study meagre sets as subsets of the real numbers, or more generally any Polish space, and reserve the term Baire space for one particular Polish space.
The complement of a meagre set is a comeagre set or residual set.
Given a topological space X, a subset A of X is meagre if it can be expressed as the union of countably many nowhere dense subsets of X. Dually, a comeagre set is one whose complement is meagre, or equivalently, the intersection of countably many sets with dense interiors.
The complement of a nowhere dense set is a dense set; in fact, it has dense interior. The complement of a dense set need not be nowhere dense; indeed, the complement can have both nowhere dense and dense regions.
A meagre set need not have measure zero.
Relation to Borel hierarchy
Just as a nowhere dense subset need not be closed, but is always contained in a closed nowhere dense subset (viz, its closure), a meagre set need not be an Fσ set (countable union of closed sets), but is always contained in an Fσ set made from nowhere dense sets (by taking the closure of each set).
Dually, just as the complement of a nowhere dense set need not be open, but has a dense interior (contains a dense open set), a comeagre set need not be a Gδ set (countable intersection of open sets), but contains a dense Gδ set formed from dense open sets.
A meagre set is also called a set of first category; a nonmeagre set (that is, a set that is not meagre) is also called a set of second category. Second category does not mean comeagre—a set may be neither meagre nor comeagre (in this case it will be of second category).
- Any subset of a meagre set is meagre; any superset of a comeagre set is comeagre.
- The union of countably many meagre sets is also meagre; the intersection of countably many comeagre sets is comeagre.
- This follows from the fact that a countable union of countable sets is countable.
Meagre sets have a useful alternative characterization in terms of the Banach–Mazur game. Let be a topological space, be a family of subsets of that have nonempty interiors such that every nonempty open set has a subset belonging to , and be any subset of . Then there is a Banach–Mazur game corresponding to . In the Banach–Mazur game, two players, and , alternately choose successively smaller elements of to produce a sequence . Player wins if the intersection of this sequence contains a point in ; otherwise, player wins. Theorem: For any meeting the above criteria, player has a winning strategy if and only if is meagre.
Subsets of the reals
- Because the rational numbers are countable, they are meagre as a subset of the reals and as a space—that is, they do not form a Baire space.
- The Cantor set is meagre as a subset of the reals, but not as a subset of itself, since it is a complete metric space and is thus a Baire space, by the Baire category theorem.
- The set of functions that have a derivative at some point is a meagre set in the space of all continuous functions.
- Oxtoby, John C. (1980). "The Banach Category Theorem". Measure and Category (Second ed.). New York: Springer. pp. 62–65. ISBN 0-387-90508-1.
- Banach, S. (1931). "Über die Baire'sche Kategorie gewisser Funktionenmengen". Studia Math. 3 (1): 174–179.