Tjyylli

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Languages that we cannot (dis)prove to be Context-Free

Base-k representations of the co-domain of a polynomial - is it context-free?For a language to be programmable, is it mandatory that it be based on a context free grammarSufficient conditions for the regularity of a context-free languageDoes there exist an extension of regular expressions that captures the context free languages?Are deterministic context-free languages closed under outfix (or other erasing operations)Is SAT a context-free language?Is equivalence of unambiguous context-free languages decidable?Example of context-free tree language which can not be generated by monadic CFTGFor which $R$ is ${0^a10^b10^cmid R(a,b,c)}$ context-free?Continuous mathematics and formal language theoryIs { ww' | HamDist(w,w')>1 } context-free?

19

5

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I'm looking for languages which are "probably not Context-Free" but we are not able to (dis)prove it using known standard techniques.

Is there a recent survey on the subject or an open problem section from a recent conference ?

Probably there are not many languages which are not known to be CF, so if you know one you can also post it as an answer.

The examples I found are:

• the well known language of Primitive words $Q = { w mid w neq u^i (|u| > 1) }$ (there's a whole nice recent book on it: Context-Free Languages and Primitive Words)


• the Base-k representations of the co-domain of a polynomial (see question "Base-k representations of the co-domain of a polynomial - is it context-free?" on cstheory, which perhaps has been solved by domotorp, see his preprint)

Note: as showed by Aryeh in his answer you can build a whole class of such languages if you "link" a language to an unknown conjecture about the (non)finiteness or (non)emptiness of some sets (e.g. $L_{Goldbach} = { 1^{2n} mid 2n$ cannot be expressed as a sum of two primes$}$). I'm not quite interested in such examples.

reference-request big-list context-free

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edited 2 hours ago

Marzio De Biasi

asked 2 days ago

Marzio De BiasiMarzio De Biasi

18.5k243114

$endgroup$

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  For your second example, I wrote a paper from my answer which is under review (and the first feedback was positive): arxiv.org/abs/1901.03913
  $endgroup$
  – domotorp
  3 hours ago
  

  
  
  
  


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  There are many variants of the first example that are not known to be context-free, I don't know if you want to include them as separate examples; see Chapter 10 of the linked book (Kászonyi-Katsura Theory).
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  – domotorp
  3 hours ago
  

  
  
  
  


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  @domotorp: I just gave it a look (I'm still reading chapter 2) ... they seem to me more technical attempts to attack the main problem.
  $endgroup$
  – Marzio De Biasi
  2 hours ago
  

  
  
  
  

add a comment | 

19

5

$begingroup$

I'm looking for languages which are "probably not Context-Free" but we are not able to (dis)prove it using known standard techniques.

Is there a recent survey on the subject or an open problem section from a recent conference ?

Probably there are not many languages which are not known to be CF, so if you know one you can also post it as an answer.

The examples I found are:

• the well known language of Primitive words $Q = { w mid w neq u^i (|u| > 1) }$ (there's a whole nice recent book on it: Context-Free Languages and Primitive Words)


• the Base-k representations of the co-domain of a polynomial (see question "Base-k representations of the co-domain of a polynomial - is it context-free?" on cstheory, which perhaps has been solved by domotorp, see his preprint)

Note: as showed by Aryeh in his answer you can build a whole class of such languages if you "link" a language to an unknown conjecture about the (non)finiteness or (non)emptiness of some sets (e.g. $L_{Goldbach} = { 1^{2n} mid 2n$ cannot be expressed as a sum of two primes$}$). I'm not quite interested in such examples.

reference-request big-list context-free

share|cite|improve this question

edited 2 hours ago

Marzio De Biasi

asked 2 days ago

Marzio De BiasiMarzio De Biasi

18.5k243114

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  For your second example, I wrote a paper from my answer which is under review (and the first feedback was positive): arxiv.org/abs/1901.03913
  $endgroup$
  – domotorp
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  There are many variants of the first example that are not known to be context-free, I don't know if you want to include them as separate examples; see Chapter 10 of the linked book (Kászonyi-Katsura Theory).
  $endgroup$
  – domotorp
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @domotorp: I just gave it a look (I'm still reading chapter 2) ... they seem to me more technical attempts to attack the main problem.
  $endgroup$
  – Marzio De Biasi
  2 hours ago
  

  
  
  
  

add a comment | 

$begingroup$

I'm looking for languages which are "probably not Context-Free" but we are not able to (dis)prove it using known standard techniques.

Is there a recent survey on the subject or an open problem section from a recent conference ?

Probably there are not many languages which are not known to be CF, so if you know one you can also post it as an answer.

The examples I found are:

• the well known language of Primitive words $Q = { w mid w neq u^i (|u| > 1) }$ (there's a whole nice recent book on it: Context-Free Languages and Primitive Words)


• the Base-k representations of the co-domain of a polynomial (see question "Base-k representations of the co-domain of a polynomial - is it context-free?" on cstheory, which perhaps has been solved by domotorp, see his preprint)

Note: as showed by Aryeh in his answer you can build a whole class of such languages if you "link" a language to an unknown conjecture about the (non)finiteness or (non)emptiness of some sets (e.g. $L_{Goldbach} = { 1^{2n} mid 2n$ cannot be expressed as a sum of two primes$}$). I'm not quite interested in such examples.

reference-request big-list context-free

share|cite|improve this question

edited 2 hours ago

Marzio De Biasi

asked 2 days ago

Marzio De BiasiMarzio De Biasi

18.5k243114

$endgroup$

share|cite|improve this question

edited 2 hours ago

Marzio De Biasi

asked 2 days ago

Marzio De BiasiMarzio De Biasi

18.5k243114

share|cite|improve this question

edited 2 hours ago

Marzio De Biasi

asked 2 days ago

Marzio De BiasiMarzio De Biasi

18.5k243114

asked 2 days ago

Marzio De BiasiMarzio De Biasi

18.5k243114

asked 2 days ago

Marzio De BiasiMarzio De Biasi

18.5k243114

2 Answers
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11

$begingroup$

How about the language $L_{TP}$ of twin primes? I.e., all pairs of natural numbers $(p,p')$ (represented, say, in unary), such that $p,p'$ are both prime and $p'=p+2$? If twin primes conjecture is true, then $L_{TP}$ is not context-free; otherwise, it's finite.

Edit: Let me give a quick proof sketch that the twin primes conjecture implies that $L_{TP}$ is not context-free. Associate to any language $L$ its length sequence $0le a_1le a_2leldots$, where the integer $ell$ appears in the sequence iff there is a word of length $ell$ in $L$. It is a consequence of the pumping lemma(s) that for $L$ that are regular or CFL, the length sequence satisfies the bounded differences property: there is an $R>0$ such that $a_{n+1}-a_nle R$ for all $n$. It is an easy and well-known fact in number theory that the primes do not have bounded differences. Finally, any infinite subsequence of a sequence violating the bounded differences property itself must violate it.

share|cite|improve this answer

edited 2 days ago

answered 2 days ago

AryehAryeh

5,89211841

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  $begingroup$
  Nice, thanks! But I'm not quite interested in languages that are linked to unknown conjectures about the (non)finiteness of some sets. BTW if those conjectures are true the resulting language is also regular :-)
  $endgroup$
  – Marzio De Biasi
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  If there are infinitely many twin primes, how do you see that $L_{TP}$ is regular?
  $endgroup$
  – Aryeh
  2 days ago
  

  
  
  
  


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  If there are infinitely many twin primes, how do you show that $L_{TP}$ is not context-free?
  $endgroup$
  – Emil Jeřábek
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
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  Oh, sorry, I didn’t notice you represent the numbers in unary. Then it is clear. (I believe that proving this for binary representation would require a considerable progress on the twin primes conjecture.)
  $endgroup$
  – Emil Jeřábek
  2 days ago
  
  
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  $begingroup$
  On the contrary, Emil, the "standard" proof that the primes in binary are not context-free easily suffices to prove that every infinite set of primes is not context-free. So if there are infinitely many twin primes, the result is immediate.
  $endgroup$
  – Jeffrey Shallit
  2 days ago
  

  
  
  
  

 | 
show 3 more comments

11

$begingroup$

Another good one is the complement of the set $S$ of contiguous subwords (aka "factors") of the Thue-Morse sequence ${bf t} = 0110100110010110 cdots $. To give some context, Jean Berstel proved that the complement of the set $T$ of prefixes of the Thue-Morse word is context-free (and actually something more general than that). But the corresponding result for subwords is still open.

share|cite|improve this answer

answered 2 days ago

Jeffrey ShallitJeffrey Shallit

6,5332636

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Great, thanks! If you saw it stated somewhere (perhaps in one of your many papers on the Thue-Morse sequence? ;-) you can add the reference (even if stated in the iterated morphism form).
  $endgroup$
  – Marzio De Biasi
  2 days ago
  

  
  
  
  

add a comment | 

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11

$begingroup$

How about the language $L_{TP}$ of twin primes? I.e., all pairs of natural numbers $(p,p')$ (represented, say, in unary), such that $p,p'$ are both prime and $p'=p+2$? If twin primes conjecture is true, then $L_{TP}$ is not context-free; otherwise, it's finite.

Edit: Let me give a quick proof sketch that the twin primes conjecture implies that $L_{TP}$ is not context-free. Associate to any language $L$ its length sequence $0le a_1le a_2leldots$, where the integer $ell$ appears in the sequence iff there is a word of length $ell$ in $L$. It is a consequence of the pumping lemma(s) that for $L$ that are regular or CFL, the length sequence satisfies the bounded differences property: there is an $R>0$ such that $a_{n+1}-a_nle R$ for all $n$. It is an easy and well-known fact in number theory that the primes do not have bounded differences. Finally, any infinite subsequence of a sequence violating the bounded differences property itself must violate it.

share|cite|improve this answer

edited 2 days ago

answered 2 days ago

AryehAryeh

5,89211841

$endgroup$

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  $begingroup$
  Nice, thanks! But I'm not quite interested in languages that are linked to unknown conjectures about the (non)finiteness of some sets. BTW if those conjectures are true the resulting language is also regular :-)
  $endgroup$
  – Marzio De Biasi
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  If there are infinitely many twin primes, how do you see that $L_{TP}$ is regular?
  $endgroup$
  – Aryeh
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  If there are infinitely many twin primes, how do you show that $L_{TP}$ is not context-free?
  $endgroup$
  – Emil Jeřábek
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Oh, sorry, I didn’t notice you represent the numbers in unary. Then it is clear. (I believe that proving this for binary representation would require a considerable progress on the twin primes conjecture.)
  $endgroup$
  – Emil Jeřábek
  2 days ago
  
  
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  $begingroup$
  On the contrary, Emil, the "standard" proof that the primes in binary are not context-free easily suffices to prove that every infinite set of primes is not context-free. So if there are infinitely many twin primes, the result is immediate.
  $endgroup$
  – Jeffrey Shallit
  2 days ago
  

  
  
  
  

 | 
show 3 more comments

11

$begingroup$

How about the language $L_{TP}$ of twin primes? I.e., all pairs of natural numbers $(p,p')$ (represented, say, in unary), such that $p,p'$ are both prime and $p'=p+2$? If twin primes conjecture is true, then $L_{TP}$ is not context-free; otherwise, it's finite.

Edit: Let me give a quick proof sketch that the twin primes conjecture implies that $L_{TP}$ is not context-free. Associate to any language $L$ its length sequence $0le a_1le a_2leldots$, where the integer $ell$ appears in the sequence iff there is a word of length $ell$ in $L$. It is a consequence of the pumping lemma(s) that for $L$ that are regular or CFL, the length sequence satisfies the bounded differences property: there is an $R>0$ such that $a_{n+1}-a_nle R$ for all $n$. It is an easy and well-known fact in number theory that the primes do not have bounded differences. Finally, any infinite subsequence of a sequence violating the bounded differences property itself must violate it.

share|cite|improve this answer

edited 2 days ago

answered 2 days ago

AryehAryeh

5,89211841

$endgroup$

• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  $begingroup$
  Nice, thanks! But I'm not quite interested in languages that are linked to unknown conjectures about the (non)finiteness of some sets. BTW if those conjectures are true the resulting language is also regular :-)
  $endgroup$
  – Marzio De Biasi
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  If there are infinitely many twin primes, how do you see that $L_{TP}$ is regular?
  $endgroup$
  – Aryeh
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  If there are infinitely many twin primes, how do you show that $L_{TP}$ is not context-free?
  $endgroup$
  – Emil Jeřábek
  2 days ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Oh, sorry, I didn’t notice you represent the numbers in unary. Then it is clear. (I believe that proving this for binary representation would require a considerable progress on the twin primes conjecture.)
  $endgroup$
  – Emil Jeřábek
  2 days ago
  
  
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  $begingroup$
  On the contrary, Emil, the "standard" proof that the primes in binary are not context-free easily suffices to prove that every infinite set of primes is not context-free. So if there are infinitely many twin primes, the result is immediate.
  $endgroup$
  – Jeffrey Shallit
  2 days ago
  

  
  
  
  

 | 
show 3 more comments

$begingroup$

How about the language $L_{TP}$ of twin primes? I.e., all pairs of natural numbers $(p,p')$ (represented, say, in unary), such that $p,p'$ are both prime and $p'=p+2$? If twin primes conjecture is true, then $L_{TP}$ is not context-free; otherwise, it's finite.

Edit: Let me give a quick proof sketch that the twin primes conjecture implies that $L_{TP}$ is not context-free. Associate to any language $L$ its length sequence $0le a_1le a_2leldots$, where the integer $ell$ appears in the sequence iff there is a word of length $ell$ in $L$. It is a consequence of the pumping lemma(s) that for $L$ that are regular or CFL, the length sequence satisfies the bounded differences property: there is an $R>0$ such that $a_{n+1}-a_nle R$ for all $n$. It is an easy and well-known fact in number theory that the primes do not have bounded differences. Finally, any infinite subsequence of a sequence violating the bounded differences property itself must violate it.

share|cite|improve this answer

edited 2 days ago

answered 2 days ago

AryehAryeh

5,89211841

$endgroup$

share|cite|improve this answer

edited 2 days ago

answered 2 days ago

AryehAryeh

5,89211841

answered 2 days ago

AryehAryeh

5,89211841

answered 2 days ago

AryehAryeh

5,89211841

11

$begingroup$

Another good one is the complement of the set $S$ of contiguous subwords (aka "factors") of the Thue-Morse sequence ${bf t} = 0110100110010110 cdots $. To give some context, Jean Berstel proved that the complement of the set $T$ of prefixes of the Thue-Morse word is context-free (and actually something more general than that). But the corresponding result for subwords is still open.

share|cite|improve this answer

answered 2 days ago

Jeffrey ShallitJeffrey Shallit

6,5332636

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Great, thanks! If you saw it stated somewhere (perhaps in one of your many papers on the Thue-Morse sequence? ;-) you can add the reference (even if stated in the iterated morphism form).
  $endgroup$
  – Marzio De Biasi
  2 days ago
  

  
  
  
  

add a comment | 

11

$begingroup$

Another good one is the complement of the set $S$ of contiguous subwords (aka "factors") of the Thue-Morse sequence ${bf t} = 0110100110010110 cdots $. To give some context, Jean Berstel proved that the complement of the set $T$ of prefixes of the Thue-Morse word is context-free (and actually something more general than that). But the corresponding result for subwords is still open.

share|cite|improve this answer

answered 2 days ago

Jeffrey ShallitJeffrey Shallit

6,5332636

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Great, thanks! If you saw it stated somewhere (perhaps in one of your many papers on the Thue-Morse sequence? ;-) you can add the reference (even if stated in the iterated morphism form).
  $endgroup$
  – Marzio De Biasi
  2 days ago
  

  
  
  
  

add a comment | 

$begingroup$

Another good one is the complement of the set $S$ of contiguous subwords (aka "factors") of the Thue-Morse sequence ${bf t} = 0110100110010110 cdots $. To give some context, Jean Berstel proved that the complement of the set $T$ of prefixes of the Thue-Morse word is context-free (and actually something more general than that). But the corresponding result for subwords is still open.

share|cite|improve this answer

answered 2 days ago

Jeffrey ShallitJeffrey Shallit

6,5332636

$endgroup$

share|cite|improve this answer

answered 2 days ago

Jeffrey ShallitJeffrey Shallit

6,5332636

answered 2 days ago

Jeffrey ShallitJeffrey Shallit

6,5332636

answered 2 days ago

Jeffrey ShallitJeffrey Shallit

6,5332636