CRM-ISM Postdoctoral Fellow€¦ · Rich Words: A Brief Overview Outline 1 Rich Words: A Brief...

Rich Words

Amy Glen

CRM-ISM Postdoctoral Fellow

LaCIM, Université du Québec à Montréal

amy.glen@gmail.comhttp://www.lacim.uqam.ca/∼glen

Le Séminaire du LaCIM

September 12, 2008

Amy Glen (LaCIM) Rich Words September 2008 1 / 22

Outline

1 Rich Words: A Brief Overview

2 Properties & Examples

3 Recent Results

4 Further Work

Rich Words: A Brief Overview

Outline

3 Recent Results

4 Further Work

What Are Rich Words?

Vague Answer: finite and infinite words that are “rich” in palindromesin the utmost sense.

A palindrome is a finite word that reads the same backwards asforwards.

English examples: eye, civic, radar, glenelg (Aussie suburb).

Droubay-Justin-Pirillo, 2001: any finite word w of length |w | containsat most |w | + 1 distinct palindromes (including the empty word ε).

G.-Justin, 2007: initiated a unified study of finite and infinite wordsthat are characterized by containing the maximal number of distinctpalindromes, called rich words.

Ambrož-Frougny-Masáková-Pelantová, 2005: independent work on “fullwords”, following earlier work of Brlek-Hamel-Nivat-Reutenauer, 2004.

Formal Definitions & Examples

Definition

A finite word w is rich iff w contains exactly |w | + 1 distinct palindromes.

Definition

Examples

abac is rich, whereas abca is not rich.

Definition

Examples

The word rich is rich . . . and poor is rich too!

Definition

Examples

But plentiful is not rich.

Definition

Examples

Definition

An infinite word is rich iff all of its factors are rich.

Definition

Examples

Definition

Examples

aω = aaaaaa · · · and abω = abbb · · · are rich.

Definition

Examples

Definition

Examples

(ab)ω = abababab · · · and (aba)ω = abaabaaba · · · are rich.

Definition

Examples

Definition

Examples

(ab)ω = abababab · · · and (aba)ω = abaabaaba · · · are rich.

abc is rich, but (abc)ω = abcabcabc · · · is not rich.

Properties & Examples

Outline

3 Recent Results

4 Further Work

Characteristic Properties

Characteristic Property 1 (Droubay-Justin-Pirillo, 2001)

A finite word w is rich if and only if every prefix (resp. suffix) of w has aunioccurrent palindromic suffix (resp. prefix).

To see this . . .

Let P(w) denote the number of palindromic factors of w .

To see this . . .

For any word u and letter x ,

P(ux) =

{P(u) if ux does not have a unioccurrent pali. suffix,P(u) + 1 if ux has a unioccurrent pali. suffix.

To see this . . .

P(ux) =

Therefore, by induction, P(w) is the number of prefixes of w thathave a unioccurrent palindromic suffix.

To see this . . .

P(ux) =

Hence P(w) ≤ |w | + 1.

To see this . . .

P(ux) =

Hence P(w) ≤ |w | + 1.

In particular P(w) = |w | + 1 (i.e., w is rich) if and only if each prefixof w has a unioccurrent palindromic suffix.

Infinite case of Characteristic Property 1:

An infinite word w is rich if and only if every prefix of w has aunioccurrent palindromic suffix.

A new palindrome is introduced at each position in a rich word.

Example: abaabaaabaaaabaaaaab · · ·

A finite or infinite word w is rich if and only if for each factor u of w , everyprefix (resp. suffix) of u has a unioccurrent palindromic suffix (resp. prefix).

Let u be a factor of a finite or infinite word w .A complete return to u in w is a factor of w having exactly twooccurrences of u, one as a prefix and one as a suffix.

Let u be a factor of a finite or infinite word w .A complete return to u in w is a factor of w having exactly twooccurrences of u, one as a prefix and one as a suffix.Example: aabcbaa is a complete return to aa in aabcbaaba (rich).

Characteristic Property 3 (G.-Justin, 2007)

A finite or infinite word w is rich if and only if for each palindromic factor p

of w , every complete return to p in w is a palindrome.

Proof:

(⇒): Suppose w is rich, but contains a non-palindromic completereturn r to a palindrome p.

Proof:

(⇒): Suppose w is rich, but contains a non-palindromic completereturn r to a palindrome p.Then r = pup for some non-palindromic word u.

Proof:

(⇒): Suppose w is rich, but contains a non-palindromic completereturn r to a palindrome p.Then r = pup for some non-palindromic word u.But then r does not have a unioccurrent palindromic suffix, acontradiction.

Proof:

(⇐): Suppose not. Let u be a factor of w of minimal length such thatu is not rich.

Proof:

(⇐): Suppose not. Let u be a factor of w of minimal length such thatu is not rich.Then u = xvy with x , y letters. By minimality xv is rich, and thelongest palindromic suffix p of u occurs more than once in u.

Proof:

(⇐): Suppose not. Let u be a factor of w of minimal length such thatu is not rich.Then u = xvy with x , y letters. By minimality xv is rich, and thelongest palindromic suffix p of u occurs more than once in u.Since all complete returns to palindromes are palindromes, we reach acontradiction to the maximality of p.Amy Glen (LaCIM) Rich Words September 2008 9 / 22

Let v denote the reversal of a word v . Example: v = abc , v = cba.

Characteristic Property 4 (Bucci-De Luca-G.-Zamboni, 2008)

For any finite or infinite word w , the following conditions are equivalent:

i) w is rich;

ii) for each factor v of w , every factor of w beginning with v and endingwith v and containing no other occurrences of v or v is a palindrome.

i) w is rich;

Proof:

i) ⇒ ii): Let u = v · · · v . If v is a palindrome, then u is a palindrome.

i) w is rich;

Proof:

i) ⇒ ii): Let u = v · · · v . If v is a palindrome, then u is a palindrome.Otherwise, for non-palindromic v , suppose u is not a palindrome . . .

i) w is rich;

Proof:

· · ·

· · · · · ·

(p is longest palindromic suffix of u)

︸︷︷︸

complete return to p → palindrome

i) w is rich;

Proof:

· · ·

· · · · · ·

(p is longest palindromic suffix of u)

︸︷︷︸

complete return to p → palindrome

. . . contradiction!

i) w is rich;

Proof:

Conversely, ii) ⇒ every complete return to a palindromic factor v

(= v) is a palindrome.

i) w is rich;

Proof:

Conversely, ii) ⇒ every complete return to a palindromic factor v

(= v) is a palindrome.

Thus w is rich by Characteristic Property 3.

Rich Examples

Purely Periodic Rich Infinite Words

(abcba)ω = abcbaabcbaabcba · · ·

Rich Examples

(aabkaabab)ω = aabkaababaabkaababaabkaabab · · · with k ≥ 0

Rich Examples

vω = vvv · · · is rich ⇔ v2 is rich

Rich Examples

vω = vvv · · · is rich ⇔ v = pq & all circular shifts are rich

Rich Examples

Other Rich Infinite Words

abcdω = abcddd · · ·

Rich Examples

aba2ba3ba4ba5b · · ·

Rich Examples

limn→∞ σn(a) = ababbababbbbababbababbbbbbbbababbaba · · ·where σ : a 7→ aba, b 7→ bb (Cassaigne, 1997).

Rich Examples

Fibonacci word: f = limn→∞ ϕn(a) = abaababaabaababaaba · · ·where ϕ : a 7→ ab, b 7→ a.

Rich Examples

Tribonacci word: r = limn→∞ θn(a) = abacabaabacababacabaaba · · ·where θ : a 7→ ab, b 7→ ac , c 7→ a.

Rich Examples

Tribonacci word: r = limn→∞ θn(a) = abacabaabacababacabaaba · · ·where θ : a 7→ ab, b 7→ ac , c 7→ a.

ψk(f) where ψk : a 7→ aabkaabab, b 7→ bab, k ≥ 0.

More General Examples

Rich words have appeared in many different contexts; they include:

Sturmian and episturmian wordsDroubay-Justin-Pirillo, 2001: characteristic property 1

Anne-Zamboni-Zorca, 2005: characteristic property 3

Bucci-De Luca-G.-Zamboni, 2008: characterization of recurrent rich infinite words

Complementation-symmetric Rote sequencesAllouche-Baake-Cassaigne-Damanik, 2003 + Bucci-De Luca-G.-Zamboni, 2008

Symbolic codings of trajectories of symmetric interval exchangetransformations – Ferenczi-Zamboni, 2008

A certain class of words associated with β-expansions where β is asimple Parry numberAmbrož-Frougny-Masáková-Pelantová, 2006 + Bucci-De Luca-G.-Zamboni, 2008

Infinite words with “abundant palindromic prefixes”Introduced by Fischler in 2006 in relation to Diophantine approximation

Basic Properties & Results (G.-Justin, 2007)

If a finite word w is rich, then its reversal w is also rich.

Example: w = aabac and w = cabaa are both rich.

If w and w ′ are rich with the same set of palindromic factors, thenthey are abelianly equivalent, i.e., |w |x = |w ′|x for all letters x .

Palindromic closure preserves richness.

The palindromic closure of a word v , denoted by v+, is the uniqueshortest palindrome beginning with v .

Examples:

Examples:(race)+ =

Examples:(race)+ = race

Examples:(race)+ = race car

(tops)+ =

(tops)+ = top s

(tops)+ = top spot

(party)+ =

(tops)+ = top spot

(party)+ = party

(tops)+ = top spot

(party)+ = party trap

(tops)+ = top spot

(tie)+ =

(tops)+ = top spot

(tie)+ = tie

(tops)+ = top spot

(tie)+ = tie it

(tops)+ = top spot

(tie)+ = tie it

(abac)+ =

(tops)+ = top spot

(tie)+ = tie it

(abac)+ = abac

(tops)+ = top spot

(tie)+ = tie it

(abac)+ = abac aba

Sturmian Words Are Rich

Theorem (de Luca, 1997)

An infinite word s over {a, b} is a standard Sturmian word if and only ifthere exists an infinite word ∆ = x1x2x3 · · · over {a, b} (not of the formuaω or ubω), called the directive word of s , such that

s = limn→∞

Pal(x1x2 · · · xn).

s = limn→∞

Pal(x1x2 · · · xn).

Pal is the iterated palindromic closure function:

Pal(ε) = ε (empty word) and Pal(wx) = (Pal(w)x)+

for any word w and letter x . Example: Pal(aba) =

s = limn→∞

Pal(x1x2 · · · xn).

for any word w and letter x . Example: Pal(aba) =a

s = limn→∞

Pal(x1x2 · · · xn).

for any word w and letter x . Example: Pal(aba) =a b

s = limn→∞

Pal(x1x2 · · · xn).

for any word w and letter x . Example: Pal(aba) =a b a

s = limn→∞

Pal(x1x2 · · · xn).

for any word w and letter x . Example: Pal(aba) =a b a a

s = limn→∞

Pal(x1x2 · · · xn).

for any word w and letter x . Example: Pal(aba) =a b a a b a

s = limn→∞

Pal(x1x2 · · · xn).

The Fibonacci word is directed by ∆ = (ab)ω = ababab · · · .

That is: f = Pal(ababab · · · ) = abaababaaba · · · .

s = limn→∞

Pal(x1x2 · · · xn).

The Fibonacci word is directed by ∆ = (ab)ω = ababab · · · .

That is: f = Pal(ababab · · · ) = abaababaaba · · · .

Palindromic closure preserves richness ⇒ Pal does too ⇒ Sturmianwords are RICH.Amy Glen (LaCIM) Rich Words September 2008 14 / 22

Episturmian Words Are Rich Too

{a, b} −→ A (finite alphabet) gives standard episturmian words.

Theorem (Droubay-Justin-Pirillo, 2001)

An infinite word s over A is a standard episturmian word if and only ifthere exists an infinite word ∆ = x1x2x3 · · · over A such that

s = limn→∞

Pal(x1x2 · · · xn).

Episturmian Words Are Rich Too

{a, b} −→ A (finite alphabet) gives standard episturmian words.

Theorem (Droubay-Justin-Pirillo, 2001)

An infinite word s over A is a standard episturmian word if and only ifthere exists an infinite word ∆ = x1x2x3 · · · over A such that

s = limn→∞

Pal(x1x2 · · · xn).

Example

∆ = (abc)ω = abcabcabc · · · directs the Tribonacci word:

r = abacabaabacababacabaabacabacabaabaca · · ·

Recent Results

Outline

3 Recent Results

4 Further Work

Recent Results

A Connection Between Palindromic & Factor Complexity

Let w be a finite or infinite word.

Recent Results

Palindromic complexity function Pw (n): counts the number of distinctpalindromic factors of w of length n for each n ∈ N.

Recent Results

Factor complexity function Cw (n): counts the number of distinctfactors of w of length n for each n ∈ N.

Recent Results

Allouche-Baake-Cassaigne-Damanik, 2003: for any aperiodic infiniteword w,

Pw(n) ≤16

⌋)for all n ∈ N.

Recent Results

Pw(n) ≤16

Baláži-Masáková-Pelantová, 2007: for any uniformly recurrent infinite wordw with F (w) closed under reversal,

Pw(n) + Pw(n + 1) ≤ Cw(n + 1) − Cw(n) + 2 for all n ∈ N. (∗)

Recent Results

Pw(n) ≤16

Baláži-Masáková-Pelantová, 2007: for any uniformly recurrent infinite wordw with F (w) closed under reversal,

Pw(n) + Pw(n + 1) ≤ Cw(n + 1) − Cw(n) + 2 for all n ∈ N. (∗)

Bucci-De Luca-G.-Zamboni, 2008: infinite words w for whichPw(n) +Pw(n + 1) reaches the upper bound in (∗) for every n are rich . . .

Recent Results

Theorem A (Bucci-De Luca-G.-Zamboni, 2008)

For any infinite word w with set of factors F (w) closed under reversal, thefollowing conditions are equivalent:

(I) all complete returns to any palindrome in w are palindromes;

(II) Pw(n) + Pw(n + 1) = Cw(n + 1) − Cw(n) + 2 for all n ∈ N.

Recent Results

Complementation-symmetric Rote sequences:

Infinite words over {a, b} with factors closed under bothcomplementation and reversal, and such that C(n) = 2n for all n.

Recent Results

Allouche-Baake-Cassaigne-Damanik, 2003: P(n) = 2 for all n.

Recent Results

Allouche-Baake-Cassaigne-Damanik, 2003: P(n) = 2 for all n.

Hence P(n) +P(n + 1) = 4 = C (n + 1)−C (n) + 2 for all n ⇒ RICH.

Recent Results

Sturmian words:

Morse-Hedlund, 1940: C(n) = n + 1 for all n

Recent Results

Sturmian words:

Droubay-Pirillo, 1999: P(n) = 1 for n even, P(n) = 2 for n odd

Recent Results

Sturmian words:

Droubay-Pirillo, 1999: P(n) = 1 for n even, P(n) = 2 for n odd

Hence P(n) + P(n + 1) = 3 = C(n + 1) − C(n) + 2 for all n ⇒ RICH.

Recent Results

Finite Case of Theorem A

Using completely different methods . . .

Theorem (de Luca-G.-Zamboni, 2008)

For any finite word w , the following two conditions are equivalent:

i) w is a rich palindrome;

ii) Pw (n) + Pw (n + 1) = Cw (n + 1) − Cw (n) + 2 for all n, 0 ≤ n ≤ |w |.

Recent Results

We also explored various interconnections between rich words,Sturmian words, and trapezoidal words.

Recent Results

A finite word w is trapezoidal if the graph of Cw (n) as a function of n

(for 0 ≤ n ≤ |w |) is that of a regular trapezoid.

Recent Results

(for 0 ≤ n ≤ |w |) is that of a regular trapezoid.Introduced by de Luca in 1999 when studying the factor complexity offinite Sturmian words.

Recent Results

Every finite Sturmian word is trapezoidal, but not conversely.E.g., aabb is trapezoidal, but not Sturmian.

Recent Results

Every finite Sturmian word is trapezoidal, but not conversely.E.g., aabb is trapezoidal, but not Sturmian.

Every trapezoidal word is rich, but not conversely. E.g., aabbaa.

Further Work

Outline

3 Recent Results

4 Further Work

Further Work

More Stuff on Rich Words

G.-Justin-Widmer-Zamboni, Palindromic richness, 2008

almost rich words: only a finite number of prefixes do not have aunioccurrent palindromic suffix

Further Work

Example: (pq)ω = pqpqpq · · · where p, q are palindromes

Further Work

weakly rich words: all complete returns to letters are palindromes

Further Work

Example: (aacbccbcacbc)ω = aacbccbcacbcaacbccbcacbc · · ·

Further Work

action of morphisms on (almost) rich words

Further Work

morphisms that preserve (almost) richness

Further Work

morphisms that preserve (almost) richness

Further Work

Characterization of morphisms that preserve (almost) richness

Enumeration of rich words

Thank You!

Dammit, I’m mad!

U R 2, R U?

* Both phrases are rich palindromes! *

CRM-ISM Postdoctoral Fellow€¦ · Rich Words: A Brief Overview Outline 1 Rich Words: A Brief...

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