Levi's lemma
In theoretical computer science and mathematics, especially in the area of combinatorics on words, the Levi lemma states that, for all strings u, v, x and y, if uv = xy, then there exists a string w such that either
- uw = x and v = wy (if |u| ≤ |x|)
or
- u = xw and wv = y (if |u| ≥ |x|)
That is, there is a string w that is "in the middle", and can be grouped to one side or the other. Levi's lemma is named after Friedrich Wilhelm Levi, who published it in 1944.[1]
Applications
Template:See Also Levi's lemma can be applied repeatedly in order to solve word equations; in this context it is sometimes called the Nielsen transformation by analogy with the Nielsen transformation for groups. For example, starting with an equation xα = yβ where x and y are the unknowns, we can transform it (assuming |x| ≥ |y|, so there exists t such that x=yt) to ytα = yβ, thus to tα = β. This approach results in a graph of substitutions generated by repeatedly applying Levi's lemma. If each unknown appears at most twice, then a word equation is called quadratic; in a quadratic word equation the graph obtained by repeatedly applying Levi's lemma is finite, so it is decidable if a quadratic word equation has a solution.[2] A more general method for solving word equations is Makanin's algorithm.[3][4]
Generalizations
The above is known as the Levi lemma for strings; the lemma can occur in a more general form in graph theory and in monoid theory; for example, there is a more general Levi lemma for traces originally due to Christine Duboc.[5] Several proofs of Levi's Lemma for traces can be found in The Book of Traces.[6]
A monoid in which Levi's lemma holds is said to have the equidivisibility property.[7] The free monoid of strings and string concatenation has this property (by Levi's lemma for strings), but by itself equidivisibility is not enough to guarantee that a monoid is free. However an equidivisible monoid M is free if additionally there exists a homomorphism f from M to the monoid of natural numbers (free monoid on one generator) with the property that the preimage of 0 contains only the identity element of M, i.e. . (Note that f simply being a homomorphism does not guarantee this latter property, as there could be multiple elements of M mapped to 0.)[8] A monoid for which such a homomorphism exists is also called graded (and the f is called a gradation).[9]