Can someone help with this:

$L=\{a^ib^j \mid i,j \ge 1 \text{ and } i \ne j \text{ and } i<2j\}$

I'm trying to write a grammar for this language? I tried this:

$S \to S_1 \mid S_2 \\ S_1 \to aXb \\ X \to aXb \mid aaXb \mid aab \\ S_2 \to aYb \\ Y \to aYb \mid Yb \mid b \\ $

What do you think?

  • 1
    $\begingroup$ Seems correct. Is there a problem? $\endgroup$ Feb 16, 2013 at 11:46
  • $\begingroup$ You should format your questions using MathJax in the future. You can learn more about the formatting by clicking on the "edit" link of a question to see its source. Most latex symbols work when used within '$' signs. You can look this up for a quick reference. $\endgroup$
    – Paresh
    Feb 16, 2013 at 13:37

1 Answer 1


The solution in the question is correct.

The constraint $i\ne j$ is the one that gives us trouble. To get around it we have to split into two cases: (i) $i<j$, and (ii) $i>j$ (but still $i<2j$)

$S\to S_{(i)} \mid S_{(ii)}$
as the first production splits into this two cases.

Now, divide and conquer: case (i) is very simple, since we only need $i<j$,
$S_{(i)} \to aS_{(i)}b \mid B$
$ B \to Bb \mid b$

This is your $S_2$.

For case (ii), we need to $j< i < 2j$, so for every single $a$, the variable $C$ will generate exactly one $b$, and at a certain point we switch to the variable $D$ that will generate two $a$'s for each $b$.
$S_{(ii)} \to aCb $
$ C \to aCb \mid D$
$ D \to aaDb \mid aab$

This is your $S_1$.

  • $\begingroup$ To get $i < j$ you need $S_{(i)} \rightarrow a S_{(i)} b \mid B b$, $B \rightarrow B b \mid \epsilon$. The $B$ alternative adds at least one $b$ here. $\endgroup$
    – vonbrand
    Mar 20, 2013 at 13:29
  • $\begingroup$ yes, you're right. $\endgroup$
    – Ran G.
    Mar 20, 2013 at 16:19

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