TY - JOUR

T1 - Words guaranteeing minimum image

AU - Margolis, S. W.

AU - Pin, J. E.

AU - Volkov, M. V.

PY - 2004

Y1 - 2004

N2 - Given a positive integer n and a finite alphabet Σ, a word w over Σ is said to guarantee minimum image if, for every homomorphism φ from the free monoid Σ* over Σ into the monoid of all transformations of an n-element set, the range of the transformation wφ has the minimum cardinality among the ranges of all transformations of the form vφ where v runs over Σ*. Although the existence of words guaranteeing minimum image is pretty obvious, the problem of their explicit description is very far from being trivial. Sauer and Stone in 1991 gave a recursive construction for such a word w but the length of their word was doubly exponential (as a function of n). We first show that some known results of automata theory immediately lead to an alternative construction that yields a simpler word that guarantees minimum image: it has exponential length, more precisely, its length is O(|Σ|(n3-n)). Then with some more effort, we find a word guaranteeing minimum image similar to that of Sauer and Stone but of length O(|Σ|(n2-n)). On the other hand, we prove that the length of any word guaranteeing minimum image cannot be less than |Σ|n-1.

AB - Given a positive integer n and a finite alphabet Σ, a word w over Σ is said to guarantee minimum image if, for every homomorphism φ from the free monoid Σ* over Σ into the monoid of all transformations of an n-element set, the range of the transformation wφ has the minimum cardinality among the ranges of all transformations of the form vφ where v runs over Σ*. Although the existence of words guaranteeing minimum image is pretty obvious, the problem of their explicit description is very far from being trivial. Sauer and Stone in 1991 gave a recursive construction for such a word w but the length of their word was doubly exponential (as a function of n). We first show that some known results of automata theory immediately lead to an alternative construction that yields a simpler word that guarantees minimum image: it has exponential length, more precisely, its length is O(|Σ|(n3-n)). Then with some more effort, we find a word guaranteeing minimum image similar to that of Sauer and Stone but of length O(|Σ|(n2-n)). On the other hand, we prove that the length of any word guaranteeing minimum image cannot be less than |Σ|n-1.

UR - http://www.scopus.com/inward/record.url?scp=11444266899&partnerID=8YFLogxK

U2 - 10.1142/S0129054104002406

DO - 10.1142/S0129054104002406

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AN - SCOPUS:11444266899

SN - 0129-0541

VL - 15

SP - 259

EP - 276

JO - International Journal of Foundations of Computer Science

JF - International Journal of Foundations of Computer Science

IS - 2

ER -