# Unavoidable pattern

In mathematics and theoretical computer science, an unavoidable pattern is a pattern of symbols that must occur in any sufficiently long string over an alphabet. An avoidable pattern is one for which there are infinitely many words no part of which match the pattern.

Let A be an alphabet of letters and E a disjoint alphabet of pattern symbols or "variables". Elements of E+ are patterns. For a pattern p, the pattern language is that subset of A containing all words h(p) where h is a non-erasing semigroup morphism from the free monoid E to A. A word w in A matches or meets p if it contains some word in the pattern language as a factor, otherwise w avoids p.[1][2]

A pattern p is avoidable on A if there are infinitely many words in A that avoid p; it is unavoidable on A if all sufficiently long words in A match p. We say that p is k-unavoidable if it is unavoidable on every alphabet of size k and correspondingly k-avoidable if it is avoidable on an alphabet of size k.[3][4]

There is a word W(k) over an alphabet of size 4k which avoids every avoidable pattern with less than 2k variables.[5]

## Examples

• The Thue–Morse sequence avoids the patterns xxx and xyxyx.[3][4]
• The patterns x and xyx are unavoidable on any alphabet.[2][6]
• The power pattern xx is 3-avoidable;[2][3] words avoiding this pattern are square-free.[4][7]
• The power patterns xn for n ≥ 3 are 2-avoidable: the Thue–Morse sequence is an example for n=3.[3]
• Sesquipowers are unavoidable.[6]

## Avoidability index

The avoidability index of a pattern p is the smallest k such that p is k-avoidable, or ∞ if p is unavoidable.[8] For binary patterns (two variables x and y) we have:[9]

• The Zimin words x, xyx, xyxzxyx, xyxzxyxwxyxzxyx, etc. are unavoidable, as well as any word that can be written as a subword of a Zimin word via a homomorphism. All other words are avoidable.
• Many patterns have avoidability index 2.
• xx,xxy,xyy,xxyx,xxyy,xyxx,xyxy,xyyx,xxyxx,xxyxy,xyxyy have avoidability index 3, though this needs a citation to verify completeness of this list.
• abwbaxbcyaczca has avoidability index 4, as well as other locked words. (Baker, McNulty, Taylor 1989)
• abvbawbcxacycdazdcd has avoidability index 5. (Clark 2004)
• no words with index greater than 5 have been found.

## Square-free words

Main article: Square-free word

A square-free word is one avoiding the pattern xx. An example is the word over the alphabet {0,±1} obtained by taking the first difference of the Thue–Morse sequence.[10][11]

## References

1. ^ Lothaire (2011) p. 112
2. ^ a b c Allouche & Shallit (2003) p.24
3. ^ a b c d Lothaire (2011) p. 113
4. ^ a b c Berstel et al (2009) p.127
5. ^ Lothaire (2011) p. 122
6. ^ a b Lothaire (2011) p.115
7. ^ Lothaire (2011) p. 114
8. ^ Lothaire (2011) p.124
9. ^ Lothaire (2011) p.126
10. ^ Pytheas Fogg (2002) p.104
11. ^ Berstel et al (2009) p.97