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In [[mathematics]], the '''Eilenberg–Mazur swindle''', named after [[Samuel Eilenberg]] and [[Barry Mazur]], is a method of proof  that involves paradoxical properties of  infinite sums. In [[geometric topology]] it was introduced by {{harvs|authorlink=Barry Mazur|last=Mazur|year=1959|year2=1961|txt=yes}} and is often called the '''Mazur swindle'''.  In algebra it was introduced by Samuel Eilenberg and is known as the  '''Eilenberg swindle''' or '''Eilenberg telescope''' (see [[telescoping sum]]).

The Eilenberg–Mazur swindle is similar to  the following well known joke "proof" that 1&nbsp;=&nbsp;0:
: 1&nbsp;=&nbsp;1&nbsp;+&nbsp;(&minus;1&nbsp;+&nbsp;1)&nbsp;+&nbsp;(&minus;1&nbsp;+&nbsp;1)&nbsp;+&nbsp;...&nbsp;=&nbsp;1&nbsp;&minus;&nbsp;1&nbsp;+&nbsp;1&nbsp;&minus;&nbsp;1&nbsp;+&nbsp;...&nbsp;=&nbsp;(1&nbsp;&minus;&nbsp;1)&nbsp;+&nbsp;(1&nbsp;&minus;&nbsp;1)&nbsp;+&nbsp;...&nbsp;=&nbsp;0
This "proof" is not valid as a claim about real numbers because [[Grandi's series]] 1&nbsp;&minus;&nbsp;1&nbsp;+&nbsp;1&nbsp;&minus;&nbsp;1&nbsp;+&nbsp;... does not converge, but the analogous argument can be used in some contexts where there is some sort of "addition" defined on some objects for which infinite sums do make sense,
to show that if ''A''&nbsp;+&nbsp;''B''&nbsp;=&nbsp;0 then ''A'' = ''B'' = 0.

==Mazur swindle==
In geometric topology the addition used in the swindle is usually the [[connected sum]] of knots or manifolds.

'''Example''' {{harv|Rolfsen|1990|loc=chapter 4B}}: A typical application of the '''Mazur swindle''' in geometric topology is the proof that the [[knot sum|sum]] of two [[trivial knot|non-trivial]] [[knot (mathematics)|knot]]s ''A'' and ''B'' is non-trivial. For knots it is possible to take infinite sums by making the knots smaller and smaller, so if ''A''&nbsp;+&nbsp;''B'' is trivial then

:<math>A=A+(B+A)+(B+A)+\cdots = (A+B)+(A+B)+\cdots=0\,</math>

so ''A'' is trivial (and ''B'' by a similar argument). The infinite sum of knots is usually a [[wild knot]], not  a [[tame knot]].
See {{harv|Poénaru|2007}} for more geometric examples.

'''Example''': The  oriented ''n''-manifolds have an addition operation given by connected sum, with 0 the ''n''-sphere. If ''A''&nbsp;+&nbsp;''B'' is the ''n''-sphere, then ''A''&nbsp;+&nbsp;''B''&nbsp;+&nbsp;''A''&nbsp;+&nbsp;''B''&nbsp;+&nbsp;... is Euclidean space so the Mazur swindle shows that the connected sum of ''A'' and Euclidean space is Euclidean space, which shows that ''A'' is the 1-point compactification of Euclidean space and therefore ''A'' is homeomorphic to the ''n''-sphere. (This does not show in the case of smooth manifolds that ''A'' is diffeomorphic to the ''n''-sphere, and in some dimensions, such as 7, there are examples of [[exotic sphere]]s ''A'' with inverses that are not diffeomorphic to the standard ''n''-sphere.)

==Eilenberg swindle==
In algebra the addition used in the swindle is usually the direct sum of [[module (mathematics)|modules]] over a [[ring (mathematics)|ring]].

'''Example:''' A typical application of the  '''Eilenberg swindle''' in algebra is the proof  that if ''A'' is a [[projective module]] over a ring ''R'' then there is a [[free module]] ''F'' with ''A''&nbsp;+&nbsp;''F''&nbsp;=&nbsp;''F''.<ref>Lam (1999), Corollary 2.7, p. 22; Eklof & Mekler (2002), Lemma 2.3, {{Google books quote|id=ltKniNkoUyUC|page=9|text=Eilenberg|p. 9}}.</ref> To see this, choose a module ''B'' such that ''A''&nbsp;+&nbsp;''B'' is free, which can be done as ''A'' is projective, and put
:''F'' = ''B'' + ''A'' + ''B'' + ''A'' + ''B'' + ....
so that
:''A'' + ''F'' = ''A'' + (''B'' + ''A'') + (''B'' + ''A'') + ... = (''A'' + ''B'') + (''A'' + ''B'') + ... = ''F''.

'''Example''': {{harv|Eisenbud|1995|loc=p.121}} Finitely generated free modules over commutative rings ''R'' have a well-defined natural number as their dimension which is additive under direct sums, and are isomorphic if and only if they have the same dimension.
This is false for some noncommutative rings, and a counterexample can be constructed using the Eilenberg swindle as follows. Let ''X'' be an abelian group such that ''X''&nbsp;=&nbsp;''X''&nbsp;+&nbsp;''X'' (for example the direct sum of an infinite number of copies of an abelian group), and let ''R'' be the ring of endomorphisms of ''X''. Then the left ''R''-module ''R'' is isomorphic to the left ''R''-module ''R''&nbsp;+&nbsp;''R''.

'''Example''': {{harv|Lam|2003|loc=Exercise 8.16}} If ''A'' and ''B'' are any groups then the Eilenberg swindle can be used to construct a ring ''R'' such that the group rings  ''R''[''A''] and ''R''[''B''] are isomorphic rings: take ''R'' to be the group ring of ''A''&nbsp;+&nbsp;''B''&nbsp;+&nbsp;''A''&nbsp;+&nbsp;''B''&nbsp;+&nbsp;...

==Other examples==

The proof of the [[Cantor–Bernstein–Schroeder theorem]] uses a similar idea. If there are injections of sets from ''X'' to ''Y'' and from ''Y'' to ''X'', this means that formally we have ''X''=''Y''+''A'' and ''Y''=''X''+''B'' for some sets ''A'' and ''B'', where + means disjoint union and = means there is a bijection between two sets. Expanding the former with the latter,
:''X'' = ''X'' + ''A'' + ''B''.
In this bijection, let ''Z'' consist of those elements of the left hand side that correspond to an element of ''X'' on the right hand side.  This bijection then expands to the bijection
:''X'' = ''A'' + ''B'' + ''A'' + ''B'' + ... + ''Z''.
Substituting the right hand side for ''X'' in ''Y'' = ''B'' + ''X'' gives the bijection
:''Y'' = ''B'' + ''A'' + ''B'' + ''A'' + ... + ''Z''.
Switching every adjacent pair ''B'' + ''A'' yields
:''Y'' = ''A'' + ''B'' + ''A'' + ''B'' + ... + ''Z''.
Composing the bijection for ''X'' with the inverse of the bijection for ''Y'' then yields
:''X'' = ''Y''.
This argument depended on the bijections ''A'' + ''B'' = ''B'' + ''A'' and ''A'' + (''B'' + ''C'') = (''A'' + ''B'') + ''C'' as well as the well-definedness of infinite disjoint union.

== Notes ==
<references/>

==References==

*{{citation|mr=0143789
|last=Bass|first= Hyman|authorlink=Hyman Bass
|title=Big projective modules are free
|journal=Illinois J. Math.|volume= 7|year=1963|pages= 24–31|url=http://projecteuclid.org/DPubS/Repository/1.0/Disseminate?view=body&id=pdf_1&handle=euclid.ijm/1255637479}}
*{{citation|mr=1322960 |authorlink=David Eisenbud|last= Eisenbud|first= David |title=Commutative algebra. With a view toward algebraic geometry|series= Graduate Texts in Mathematics|volume= 150|publisher=Springer-Verlag|publication-place= New York |year=1995|pages= xvi+785 | isbn= 0-387-94268-8}}
*{{citation|first1=Paul C. |last1=Eklof|first2=Alan H.|last2=Mekler|title=Almost free modules: set-theoretic models|year=2002|isbn=0-444-50492-3|publisher=Elsevier}}
*{{citation|first=Tsit-Yuen |last=Lam|title= Exercises in Classical Ring Theory|year=2003|isbn= 978-0-387-00500-3|publisher=Springer|location=New York, NY}}
*{{citation|first=Tsit-Yuen |last=Lam|title=Lectures on modules and rings|year=1999|isbn=0-387-98428-3|publisher=Springer}}
*{{citation|last=Mazur|first= Barry
|title=On the structure of certain semi-groups of spherical knot classes|journal= Publications Mathématiques de l'IHÉS|volume= 3 |year=1959|pages= 19–27
|mr=0116347|url= http://www.numdam.org/item?id=PMIHES_1959__3__19_0}}
*{{citation|mr=0125570|last=Mazur|first= B. C.|title= On embeddings of spheres|journal=  Acta Math. |volume= 105 |year= 1961 |pages=1–17|doi=10.1007/BF02559532}}
*{{citation|mr=2311984
|last=Poénaru|first= Valentin|authorlink=Valentin Poénaru
|title=What is ... an infinite swindle?
|journal=Notices Amer. Math. Soc.|volume= 54 |year=2007|issue= 5|pages= 619–622|url=http://www.ams.org/notices/200705/what-is-poenaru-web.pdf}}
*{{citation|mr=1277811|last= Rolfsen|first=Dale |title=Knots and links. Corrected reprint of the 1976 original. |series=Mathematics Lecture Series|volume= 7|publisher= Publish or Perish, Inc.|publication-place= Houston, TX,|year= 1990|pages= xiv+439 | isbn= 0-914098-16-0}}

==External links==
*[http://math.berkeley.edu/~stall/mazur.html Talk by John Stallings] on Mazur's swindle in topology
*[http://terrytao.wordpress.com/2009/10/05/mazurs-swindle/ Exposition by Terence Tao] on Mazur's swindle in topology

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[[Category:Knot theory]]
[[Category:Module theory]]