In mathematics, a subset of a topological space is called nowhere dense or rare if its closure has empty interior.[note 1] In a very loose sense, it is a set whose elements are not tightly clustered (as defined by the topology on the space) anywhere. For example, the integers are nowhere dense among the reals, whereas an open ball is not.
The surrounding space matters: a set may be nowhere dense when considered as a subset of a topological space but not when considered as a subset of another topological space Notably, a set is always dense in its own subspace topology.
Density nowhere can be characterized three different (but equivalent) ways. The simplest definition is the one from density:
Expanding out the negation of density, it is equivalent to require that each open set contains an open subset disjoint from  It suffices to check either condition on a base for the topology on and density nowhere in is often described as being dense in no open interval.
Definition by closure
The second definition above is equivalent to requiring that the closure, cannot contain any open set. This is the same as saying that the interior of the closure of (both taken in )[note 1] is empty; that is,
Definition by boundaries
From the previous remark, is nowhere dense in if and only if is a subset of the boundary of a dense open subset: namely, In fact, one can remove the denseness condition:
is nowhere dense iff there exists some open subset of such that
Alternatively, one can strengthen the containment to equality by taking the closure:
is nowhere dense iff there exists some open subset of such that 
Properties and sufficient conditions
- A set is nowhere dense iff its closure is. Thus a nowhere dense set need not be closed (for instance, the set is nowhere dense in the reals), but is properly contained in a nowhere dense closed set.
- If is nowhere dense in then is nowhere dense in
- If is nowhere dense in and is an open subset of then is nowhere dense in 
- Every subset of a nowhere dense set is nowhere dense.
- The union of finitely many nowhere dense sets is nowhere dense.
The union of countably many nowhere dense sets, however, need not be nowhere dense. (Thus, the nowhere dense sets do not, in general, form a sigma-ideal.) Instead, such a union is called a meagre set or a set of first category.
- is nowhere dense in : although the points get arbitrarily close to the closure of the set is which has empty interior (and is thus also nowhere dense in ).
- is nowhere dense in 
- is nowhere dense in but the rationals are not (they are dense everywhere).
- is not nowhere dense in : it is dense in the interval and in particular the interior of its closure is
- The empty set is nowhere dense. In a discrete space, the empty set is the only such subset.
- In a T1 space, any singleton set that is not an isolated point is nowhere dense.
- The boundary of every open set and of every closed set is nowhere dense.
- A vector subspace of a topological vector space is either dense or nowhere dense.
Nowhere dense sets with positive measure
A nowhere dense set is not necessarily negligible in every sense. For example, if is the unit interval not only is it possible to have a dense set of Lebesgue measure zero (such as the set of rationals), but it is also possible to have a nowhere dense set with positive measure.
For one example (a variant of the Cantor set), remove from all dyadic fractions, i.e. fractions of the form in lowest terms for positive integers and the intervals around them: Since for each this removes intervals adding up to at most the nowhere dense set remaining after all such intervals have been removed has measure of at least (in fact just over because of overlaps) and so in a sense represents the majority of the ambient space This set is nowhere dense, as it is closed and has an empty interior: any interval is not contained in the set since the dyadic fractions in have been removed.
Generalizing this method, one can construct in the unit interval nowhere dense sets of any measure less than although the measure cannot be exactly 1 (because otherwise the complement of its closure would be a nonempty open set with measure zero, which is impossible).
For another simpler example, if is any dense open subset of having finite Lebesgue measure then is necessarily a closed subset of having infinite Lebesgue measure that is also nowhere dense in (because its topological interior is empty). Such a dense open subset of finite Lebesgue measure is commonly constructed when proving that the Lebesgue measure of the rational numbers is This may be done by choosing any bijection (it actually suffices for to merely be a surjection) and for every letting
- Baire space
- Fat Cantor set
- Meagre set – A notion in general topology of a "small" or "negligible" subset of a topological space
- Narici & Beckenstein 2011, pp. 371-423.
- Fremlin 2002, 3A3F(a).
- Oxtoby, John C. (1980). Measure and Category (2nd ed.). New York: Springer-Verlag. pp. 1–2. ISBN 0-387-90508-1.
A set is nowhere dense if it is dense in no interval; although note that Oxtoby later gives the interior-of-closure definition on page 40.
- Natanson, Israel P. (1955). Teoria functsiy veshchestvennoy peremennoy [Theory of functions of a real variable]. Volume I (Chapters 1-9). Translated by Boron, Leo F. New York: Frederick Ungar. p. 88. hdl:2027/mdp.49015000681685. LCCN 54-7420.
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- Steen, Lynn Arthur; Seebach Jr., J. Arthur (1995). Counterexamples in Topology (Dover republication of Springer-Verlag 1978 ed.). New York: Dover. p. 7. ISBN 978-0-486-68735-3.
A subset of is said to be nowhere dense in if no nonempty open set of is contained in
- Gamelin, Theodore W. (1999). Introduction to Topology (2nd ed.). Mineola: Dover. pp. 36–37. ISBN 0-486-40680-6 – via ProQuest ebook Central.
- Rudin 1991, p. 41.
- Willard, Stephen (1970). General topology. Reading, Mass.: Addison-Wesley. p. 37. hdl:2027/mdp.49015000696204. LCCN 74-100890.CS1 maint: date and year (link) See 4G(2-3).
- Fremlin 2002, 3A3F(c).
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