I have to find a Context-Free grammar that generates the language:

$L_1 = \{x\#y\ |\ y$ is a subsequence of $x^R$, and $x\in\{a,b\}^*\}$, $\Sigma=\{a,b,\#\}$

The concatenation of two mutually reversed strings are pretty simple, but I just can't figure out how to express "plugging in random terminals in $x$" so that $x^R$ could contain $y$ as its subsequence.


Let me start with a grammar for the language of words of the form $x\#x^R$: $$ S \to aSa \mid bSb \mid \# $$ This is our starting point. Now there are two interpretations of the term subsequence. Under the first interpretation, a subsequence is obtained by removing any number of symbols. This just means that we can omit the second side of each production of the form $S \to \sigma S \sigma$, resulting in the grammar $$ S \to aSa \mid bSb \mid aS \mid bS \mid \# $$ Under the second interpretation, the subsequence must be contiguous. Each derivation should therefore be in three steps: first only on the left side, then on both sides, then finally again only on the left side. This requires three nonterminals, and results in the following grammar: $$ S \to aS \mid bS \mid T \\ T \to aTa \mid bTb \mid U \\ U \to aU \mid bU \mid \# $$

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    $\begingroup$ Comments are not for extended discussion; this conversation about the definition of the word subsequence has been moved to chat. $\endgroup$ – Gilles 'SO- stop being evil' Aug 6 '19 at 21:14

"Plugging in random terminals in $x$" is the right approach.

To implement that, we need to understand clearly how we have achieved "the concatenation of two mutually reversed strings" first. It is done by growing a string from both ends with the same symbol repeatedly, a characteristic technique for context-free grammar, as illustrated below.

$$\begin{array}{ccc} &\#&\\ &a\#a&\\ &ba\#ab&\\ &bba\#abb\\ &abba\#abba\\ &babba\#abbab \end{array}$$

To plugging in random terminals on the left hand side, we can grow the left hand side without growing the right hand side.

Starting from $\#$, we will either add the same symbol to both side, or add a symbol to the left hand side only. Hence the following simple grammar.

$$S\to\#\mid aSa\mid bSb\mid aS\mid bS$$

Here is an easy exercise.

Exercise. find a context-free grammar that generates this language, $\{xy\mid x,y\in\{a,b\}^*$ $\text{where }x$ $\text{with some }$$a\text{'s removed is the same as}$ $y\text{ with some}$ $b\text{'s removed}\}$.

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Here's how I'd approach this:

  • Make a grammar that can produce palindromes
  • Make a grammar that can produce "any sequence of as and bs followed by #"
  • Make a grammar that can produce "any sequence of as and bs"

Then combine the three. $L_1$ is:

  • Any sequence of as and bs
  • The first half of a palindrome
  • Any sequence of as and bs followed by #
  • The second half of the palindrome

Giving something like this:

S → G P ; gibberish, followed by modified palindrome

P → a P a ; standard palindrome grammar
P → b P b
P → G # ; but instead of ∅ in its center, it has gibberish followed by #

G → a G ; produces any sequence of a's and b's
G → b G
G → ∅

(There are a few different notations for CFGs, so in case this isn't the one you're used to: S is always the starting symbol, means the empty string, and everything after a ; is a "comment" that doesn't affect the grammar.)

EDIT: It's been pointed out that you want a reversed subsequence, not a reversed substring. But this is easy to fix: just allow adding arbitrary gibberish on the left side of the "palindrome" part.

P → G P

You can then simplify the starting rule:

S → P

Everything else should remain the same.

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    $\begingroup$ "Subsequence" usually means "a sequence that can be derived from another sequence by deleting some or no elements without changing the order of the remaining elements." (Quote taken from Wikipedia.) $\endgroup$ – rici Aug 6 '19 at 3:59
  • $\begingroup$ @rici Ah, I read "substring"! My bad, I'll correct my grammar. $\endgroup$ – Draconis Aug 6 '19 at 15:40

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