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'''Schröder–Bernstein''' (or Cantor–Schröder–Bernstein, or Cantor–Bernstein) '''property''' is a collective term for mathematical properties structurally similar to the [[Schröder-Bernstein theorem]] of set theory. It is not a well-defined mathematical notion but the following pattern noted in several well-defined notions:
:If ''X'' is similar to a part of ''Y'' and also ''Y'' is similar to a part of ''X'' then ''X'' and ''Y'' are similar (to each other).


==The general pattern==
A mathematical property is said to be a ''Schröder–Bernstein property'' if it is formulated in the following form.
:If ''X'' is similar to a part of ''Y'' and also ''Y'' is similar to a part of ''X'' then ''X'' and ''Y'' are similar (to each other).
In order to be specific one should decide
* what kind of mathematical objects are ''X'' and ''Y'',
* what is meant by "a part",
* what is meant by "similar".
A Schröder–Bernstein property is a joint property of
* a class of objects,
* a binary relation "be a part of",
* a binary relation "be similar".
Instead of the relation "be a part of" one may use a binary relation "be embeddable into" interpreted as "be similar to some part of". Then a Schröder–Bernstein property takes the following form.
:If ''X'' is embeddable into ''Y'' and ''Y'' is embeddable into ''X'' then ''X'' and ''Y'' are similar.
The same in the language of [[category theory]]:
:If objects ''X'', ''Y'' are such that ''X'' injects into ''Y'' (more formally, there exists a monomorphism from ''X'' to ''Y'') and also ''Y'' injects into ''X'' then ''X'' and ''Y'' are isomorphic (more formally, there exists an isomorphism from ''X'' to ''Y'').
The relation "injects into" is a [[Order (relation)#Partial order|preorder]] (that is, reflexive and [[Transitive relation|transitive]] relation), and "isomorphic" is an [[equivalence relation]]. Also the relation "embeddable into" is usually a preorder, and similarity is usually an equivalence relation (which is natural, but not provable in the absence of formal definition). Generally, a preorder leads to an equivalence relation and a partial order between the corresponding equivalence classes. The Schröder–Bernstein property claims that the embeddability preorder (assuming that it is a preorder) leads to the similarity equivalence relation, and an order between classes of similar objects.
Not all statements of this form are true (see "Examples" below).
A problem of deciding, whether a Schröder–Bernstein property (for a given class and two relations) holds or not, is called a Schröder–Bernstein problem. A theorem that states a Schröder–Bernstein property (for a given class and two relations), thus solving the Schröder–Bernstein problem in the affirmative, is called a Schröder–Bernstein theorem (for the given class and two relations), not to be confused with the classical (Cantor–)Schröder–Bernstein theorem.
==Examples==
In the [[Schröder-Bernstein theorem|classical (Cantor-)Schröder–Bernstein theorem]],
* objects are [[Set (mathematics)|sets]] (maybe infinite),
* "a part" is interpreted as a [[subset]],
* "similar" is interpreted as [[Bijective function#Bijections and the concept of cardinality|equinumerous]].
A Schröder–Bernstein property can fail. For example, assume that
* objects are [[triangle]]s,
* "a part" means a triangle inside the given triangle,
* "similar" is interpreted as usual in elementary geometry: triangles related by a dilation (in other words, "triangles with the same shape up to a scale factor", or equivalently "triangles with the same angles").
Then the statement fails badly: every triangle ''X'' evidently is similar to some triangle inside ''Y'', and the other way round; however, ''X'' and ''Y'' need no be similar.
The Schröder–Bernstein theorem for [[measurable space]]s<ref>{{harvnb|Srivastava|1998}}, see Proposition 3.3.6 (on page 96), and the first paragraph of Section 3.3 (on page 94).</ref> states the Schröder–Bernstein property for the following case:
* objects are measurable spaces,
* "a part" is interpreted as a measurable subset treated as a measurable space,
* "similar" is interpreted as isomorphic.
It has a noncommutative counterpart, the Schröder–Bernstein theorem for operator algebras;<ref>{{harvnb|Kadison|Ringrose|1986}}, see Proposition 6.2.4 (on page 406).</ref> here
* objects are projections in a given Von Neumann algebra;
* "a part" is interpreted as a subprojection (that is, ''E'' is a part of ''F'' if ''F''–''E'' is a projection);
* "''E'' is similar to ''F''" means that ''E'' and ''F'' are the initial and final projections of some partial isometry in the algebra (that is, ''E'' = ''V*V'' and ''F'' = ''VV*'' for some ''V'' in the algebra).
[[Banach space]]s violate the Schröder–Bernstein property;<ref name=Ca>{{harvnb|Casazza|1989}}</ref><ref name=Go>{{harvnb|Gowers|1996}}</ref> here
* objects are Banach spaces,
* "a part" is interpreted as a subspace<ref name="Ca" /> or a complemented subspace<ref name=Go />,
* "similar" is interpreted as linearly homeomorphic.
Many other Schröder–Bernstein problems related to various [[space (mathematics)|spaces]] and algebraic structures (groups, rings, fields etc) are discussed by informal groups of mathematicians (see the [[Schröder–Bernstein property/External Links|external links page]]).
==Notes==
{{reflist}}
==References==
{{Citation
| last = Srivastava
| first = S.M.
| title = A Course on Borel Sets
| year = 1998
| publisher = Springer
| isbn = 0387984127
}}.
{{Citation
| last1 = Kadison
| first1 = Richard V.
| last2 = Ringrose
| first2 = John R.
| title = Fundamentals of the theory of operator algebras
| volume = II
| year = 1986
| publisher = Academic Press
| isbn = 0-12-393302-1
}}.
{{Citation
| last = Gowers
| first = W.T.
| year = 1996
| title = A solution to the Schroeder-Bernstein problem for Banach spaces
| journal = Bull. London Math. Soc.
| volume = 28
| pages = 297–304
| url = http://blms.oxfordjournals.org/content/28/3/297
}}.
{{Citation
| last = Casazza
| first = P.G.
| year = 1989
| title = The Schroeder-Bernstein property for Banach spaces
| journal = Contemp. Math.
| volume = 85
| pages = 61–78
| url = http://www.ams.org/mathscinet-getitem?mr=983381
}}.
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