Monoid factorisation
In mathematics, a factorisation of a free monoid is a sequence of subsets of words with the property that every word in the free monoid can be written as a concatenation of elements drawn from the subsets. The Chen–Fox–Lyndon theorem states that the Lyndon words furnish a factorisation. The Schützenberger theorem relates the definition in terms of a multiplicative property to an additive property.[clarification needed] Let A* be the free monoid on an alphabet A. Let Xi be a sequence of subsets of A* indexed by a totally ordered index set I. A factorisation of a word w in A* is an expression
- [math]\displaystyle{ w = x_{i_1} x_{i_2} \cdots x_{i_n} \ }[/math]
with [math]\displaystyle{ x_{i_j} \in X_{i_j} }[/math] and [math]\displaystyle{ i_1 \ge i_2 \ge \ldots \ge i_n }[/math]. Some authors reverse the order of the inequalities.
Chen–Fox–Lyndon theorem
A Lyndon word over a totally ordered alphabet A is a word that is lexicographically less than all its rotations.[1] The Chen–Fox–Lyndon theorem states that every string may be formed in a unique way by concatenating a lexicographically non-increasing sequence of Lyndon words. Hence taking Xl to be the singleton set {l} for each Lyndon word l, with the index set L of Lyndon words ordered lexicographically, we obtain a factorisation of A*.[2] Such a factorisation can be found in linear time and constant space by Duval's algorithm.[3] The algorithm[4] in Python code is:
def chen_fox_lyndon_factorization(s): n = len(s) factorization = [] i = 0 while i < n: j, k = i + 1, i while j < n and s[k] <= s[j]: if s[k] < s[j]: k = i else: k += 1 j += 1 while i <= k: factorization.append(s[i:i + j - k]) i += j - k return factorization
Hall words
The Hall set provides a factorization.[5] Indeed, Lyndon words are a special case of Hall words. The article on Hall words provides a sketch of all of the mechanisms needed to establish a proof of this factorization.
Bisection
A bisection of a free monoid is a factorisation with just two classes X0, X1.[6]
Examples:
- A = {a,b}, X0 = {a*b}, X1 = {a}.
If X, Y are disjoint sets of non-empty words, then (X,Y) is a bisection of A* if and only if[7]
- [math]\displaystyle{ YX \cup A = X \cup Y \ . }[/math]
As a consequence, for any partition P, Q of A+ there is a unique bisection (X,Y) with X a subset of P and Y a subset of Q.[8]
Schützenberger theorem
This theorem states that a sequence Xi of subsets of A* forms a factorisation if and only if two of the following three statements hold:
- Every element of A* has at least one expression in the required form;[clarification needed]
- Every element of A* has at most one expression in the required form;
- Each conjugate class C meets just one of the monoids Mi = Xi* and the elements of C in Mi are conjugate in Mi.[9][clarification needed]
See also
References
- ↑ Lothaire (1997) p.64
- ↑ Lothaire (1997) p.67
- ↑ Duval, Jean-Pierre (1983). "Factorizing words over an ordered alphabet". Journal of Algorithms 4 (4): 363–381. doi:10.1016/0196-6774(83)90017-2..
- ↑ "Lyndon factorization - Algorithms for Competitive Programming". https://cp-algorithms.com/string/lyndon_factorization.html.
- ↑ Guy Melançon, (1992) "Combinatorics of Hall trees and Hall words", Journal of Combinatoric Theory, 59A(2) pp. 285–308.
- ↑ Lothaire (1997) p.68
- ↑ Lothaire (1997) p.69
- ↑ Lothaire (1997) p.71
- ↑ Lothaire (1997) p.92
- Lothaire, M. (1997), Combinatorics on words, Encyclopedia of Mathematics and Its Applications, 17, Perrin, D.; Reutenauer, C.; Berstel, J.; Pin, J.-É.; Pirillo, G.; Foata, D.; Sakarovitch, J.; Simon, I.; Schützenberger, M. P.; Choffrut, C.; Cori, R.; Lyndon, R.; Rota, G.-C. Foreword by Roger Lyndon (2nd ed.), Cambridge University Press, ISBN 0-521-59924-5
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