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 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. The Eilenberg–Mazur swindle is similar to the following well known joke "proof" that 1 = 0: This "proof" is not valid as a claim about real numbers because Grandi's series 1 − 1 + 1 − 1 + ... 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 + B = 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 : A typical application of the Mazur swindle in geometric topology is the proof that the sum of two non-trivial knots 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 + B is trivial then so A is trivial. The infinite sum of knots is usually a wild knot, not a tame knot. See for more geometric examples. Example: The oriented n-manifolds have an addition operation given by connected sum, with 0 the n-sphere. If A + B is the n-sphere, then A + B + A + B + ... 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.
Eilenberg swindle
In algebra the addition used in the swindle is usually the direct sum of modules over a ring. Example: A typical application of the Eilenberg swindle in algebra is the proof that if A is a projective module over a ringR then there is a free moduleF with A ⊕ F ≅ F. To see this, choose a module B such that A ⊕ B is free, which can be done as A is projective, and put so that Example: Finitely generatedfree modules over commutative ringsR 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 ≅ X ⊕ X, and let R be the ring of endomorphisms of X. Then the left R-module R is isomorphic to the left R-module R ⊕ R. Example: If A and B are any groups then the Eilenberg swindle can be used to construct a ringR such that the group rings R and R are isomorphic rings: take R to be the group ring of the restricted direct product of infinitely many copies of A ⨯ B.
Other examples
The proof of the Cantor–Bernstein–Schroeder theorem might be seen as antecedent of the Eilenberg–Mazur swindle. In fact, the ideas are quite similar. 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, In this bijection, let Z consist of those elements of the left hand side that correspond to an element ofX on the right hand side. This bijection then expands to the bijection Substituting the right hand side for X in Y = B + X gives the bijection Switching every adjacent pair B + A yields Composing the bijection for X with the inverse of the bijection for Y then yields This argument depended on the bijections A + B = B + A and A + = + C as well as the well-definedness of infinite disjoint union.
