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I know this isn't directly related to programming, but I was wondering if anyone know how to apply the pumping lemma to the following proof:

Show that L={(a^n)(b^n)(c^m) : n!=m} is not a context free language

I'm pretty confident with applying pumping lemmas, but this one is really irking me. What do you think?

+2  A: 

Edit: I was totally leading you down the wrong track. That's what happens when I try to help out when I haven't completely solved the problem myself.

Ogden's Lemma

Suppose L is context free. By Ogden's lemma, there exists an integer p that has the following properties:

Given a string w in L at least p symbols long, where at least p of those symbols are "marked", w can be represented as uvxyz, which satisfy:

  1. x has at least one marked symbol,
  2. either u and v both have marked symbols or y and z both have marked symbols,
  3. vxy has at most p marked symbols, and
  4. u vi x yi z is in L for i >= 0

That's Ogden's lemma. Now, let q be an integer divisible by every positive integer no greater than p. Let w = ap+q bp+q cp. Mark every c. By #2, u or v must contain at least one c. If either u or v contains any other symbol, then #4 fails, so u and v must contain only c. But then #4 fails when i = q/|uv|. We know q is divisible by |uv| because p > |uv| > 0, and q is divisible by all positive integers less than p.

Note that Ogden's lemma turns into the pumping lemma when you mark all symbols.

Pumping Lemma

Suppose L is context free. By the pumping lemma, there is a length p (not necessarily the same p as above) such that any string w in L can be represented as uvxyz, where

  1. |vxy| <= p,
  2. |vy| >= 1, and
  3. u vi x yi z is in L for i >= 0.

Given a string w in L, either m > n or m < n. Suppose p = 2.

Suppose that m > n. (Note that Λ denotes the empty string.)

  • Let u = an bn cm-1
  • Let v = c
  • Let x = Λ
  • Let y = Λ
  • Let z = Λ

Suppose that n > m.

  • Let u = an-1
  • Let v = a
  • Let x = Λ
  • Let y = b
  • Let z = bn-1 cm

This demonstrates that no string from L provides a counterexample using the pumping lemma to the supposition that L is a context free language (even though it is context sensitive).

Dietrich Epp
I don't follow...though this would prevent the pumped selection from including all 3 characters, it wouldn't stop the selected pump section from being all C's, which necessarily be pumped to equal n
Maixy
I'm sorry, I misread the pumping lemma when I looked it up. I believe this requires Ogden's lemma, which is stronger. Fixed.
Dietrich Epp
'requires Ogden's lemma' in that it's not provable w/ the pumping lemma?
Maixy
I put up my proof with Ogden's lemma. Sorry for the unhelpful hints earlier.
Dietrich Epp
So ogden's lemma is the only choice? Thanks a bunch for the insight!
Maixy