I need to decide whether the language L = {wxw| x, w ∈ {a,b}* , w≠ε} is regular or not.
I know that L2 = {wxwR| x, w ∈ {a,b}* , w≠ε} is regular, since you can make sure that the word start and ends in the same letter, but it doesn't seem to work without the reversing, (for example w = 10, x = ε)
How do I prove it though?
It is not regular. Use Myhill-Nerode. Consider the prefix a^n b. The shortest string ending with b that can be appended to this to get a string in the language is a^n b. This means every such prefix is distinguishable and there's no DFA.
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My Argument/Answer is if y is aregular set then there exits a DFA which accepts y. In the L1 there is a condition that y=x^n, that x will belong to L1,as y is accepted by DFA. So is x^n and so is x so L1 is regular. Now L2 --> here the condition is x=y^n. Here y is accepted by DFA so is y^n so which is equal to x so x can be accepted by DFA. This makes both L1,L2 regular
Is my argument right?
This question seems like it's poorly-posed. For example, if we take A = {a}, then L1 is the language {a} and L2 is the language a*, both of which are regular. If we pick A = a*b, then L1 = a *b (which is regular) and L2 = { (anb)m | m, n ≥ 0 }, which is not regular (using the pumping lemma). In other words, the answer depends on the choice of A.
This is weird but by pumping lemma, say
Let L be a regular language. There exists a constant n such that for every string w in L such that |w| >= n, we can break w in to xyz such that xy*z is also in L.
This lemma is strong because it argues for all regular languages. But what if the regular language L = a? There is only one word (a) in it. How the pumping lemma works for this case?
If n = 2 then it is vacuously true that any w in L with |w| >= n satisfies the conclusion of the pumping lemma. No words in L are long enough to serve as counterexamples. More generally, if L is any finite language then L satisfies the pumping lemma: just take n to be greater than the length of the longest word in L.
Ok, I know that this isn't a programming question but it is a computing question so it is relevant.
Basically, how can I use the pumping lemma to prove that this language is not regular?
{w in {0,1}* | if the length of w is odd then the middle symbol is 0}
Please answer this as simple as possible as whilst I know about models of computation, I am relatively new to it.
Thank you very much in advance!
According to the pumping lemma, if that language is regular then there must exist a number p such that for all strings longer than p in the language, we can decompose that string into x + y + z, where each of x, y, and z are strings and |y| >= 1, |x + y| <= p, and x + (y * i) + z is in the language for all non-negative integers i.
Now observe that for every non-negative integer i, the string "1" * i + "0" + "1" * i is in the language. (That is, the string of i 1s followed by a single 0 and then i more 1s)
Specifically, the string S consisting of p 1s followed by a 0 and then p more 1s is in the language. Since this string has length 2 p + 1, this string is long enough that it can be broken into three strings x, y, and z as in the pumping lemma. Since |x + y| <= p, it must be that x and y are all 1s, and the only 0 character in S is in z. Now consider the string S' = x + y + y + y + z. Since we added 2*|y| characters to it, S' must also have an odd length. But we added some number of 1 characters to the left of the only 0 in S, and didn't add any 1 characters to the right of the 0. So S' doesn't have a 0 as its middle character, and therefore S' isn't in the language.
Therefore, we've shown that the language can't be pumped as the pumping lemma requires. Therefore, the language is not regular.
Pumping lemma definition (from wiki)
Let L be a regular language. Then there exists an integer p ≥ 1 depending only on L such that every string w in L of length at least p (p is called the "pumping length"[4]) can be written as w = xyz (i.e., w can be divided into three substrings), satisfying the following conditions:
|y| ≥ 1;
|xy| ≤ p
for all i ≥ 0, xyiz ∈ L
Suppose I want to test regular expression 011
Since it is regular expressionm, there is string w for at least length p that satisfy w=xyz
The number of this automata is 3, p should be >= 3
But only string that accept this automata is 011
So I pick 011 as w
I can break up 3 part 011 = xyz
but how can I break? I cannot satisfy
|y| ≥ 1;
|xy| ≤ p
for all i ≥ 0, xyiz ∈ L
Since it is only accept 011
How can I pump? Where am I wrong
Let p be 4. Then there are no strings w in L of length at least p, so any statement of the form "Every string w in L of length at least p […]" will be vacuously true. So the pumping lemma is satisfied.
Pumping lemma is generally applicable to infinite regular languages.
And is not used to prove L is regular
It is used to prove L is not regular
But it satisfies for all infinite regular languages
I'm learning the difference between the lemmata in the question. Every reference I can find uses the example:
{(a^i)(b^j)(c^k)(d^l) : i = 0 or j = k = l}
to show the difference between the two. I can find an example using the regular lemma to "disprove" it.
Select w = uvxyz, s.t. |vy| > 0, |vxy| <= p.
Suppose w contains an equal number of b's, c's, d's.
I selected:
u,v,x = ε
y = (the string of a's)
z = (the rest of the string w)
Pumping y will just add to the number of a's, and if |b|=|c|=|d| at first, it still will now.
(Similar argument for if w has no a's. Then just pump whatever you want.)
My question is, how does Ogden's lemma change this strategy? What does "marking" do?
Thanks!
One important stumbling issue here is that "being able to pump" does not imply context free, rather "not being able to pump" shows it is not context free. Similarly, being grey does not imply you're an elephant, but being an elephant does imply you're grey...
Grammar context free => Pumping Lemma is definitely satisfied
Grammar not context free => Pumping Lemma *may* be satisfied
Pumping Lemma satisfied => Grammar *may* be context free
Pumping Lemma not satisfied => Grammar definitely not context free
# (we can write exactly the same for Ogden's Lemma)
# Here "=>" should be read as implies
That is to say, in order to demonstrate that a language is not context free we must show it fails(!) to satisfy one of these lemmata. (Even if it satisfies both we haven't proved it is context free.)
Below is a sketch proof that L = { a^i b^j c^k d^l where i = 0 or j = k = l} is not context free (although it satisfies The Pumping Lemma, it doesn't satisfy Ogden's Lemma):
Pumping lemma for context free grammars:
If a language L is context-free, then there exists some integer p ≥ 1 such that any string s in L with |s| ≥ p (where p is a pumping length) can be written as
s = uvxyz
with substrings u, v, x, y and z, such that:
1. |vxy| ≤ p,
2. |vy| ≥ 1, and
3. u v^n x y^n z is in L for every natural number n.
In our example:
For any s in L (with |s|>=p):
If s contains as then choose v=a, x=epsilon, y=epsilon (and we have no contradiction to the language being context-free).
If s contains no as (w=b^j c^k d^l and one of j, k or l is non-zero, since |s|>=1) then choose v=b (if j>0, v=c elif k>0, else v=c), x=epsilon, y=epsilon (and we have no contradiction to the language being context-free).
(So unfortunately: using the Pumping Lemma we are unable to prove anything about L!
Note: the above was essentially the argument you gave in the question.)
Ogden's Lemma:
If a language L is context-free, then there exists some number p > 0 (where p may or may not be a pumping length) such that for any string w of length at least p in L and every way of "marking" p or more of the positions in w, w can be written as
w = uxyzv
with strings u, x, y, z, and v such that:
1. xz has at least one marked position,
2. xyz has at most p marked positions, and
3. u x^n y z^n v is in L for every n ≥ 0.
Note: this marking is the key part of Ogden's Lemma, it says: "not only can every element be "pumped", but it can be pumped using any p marked positions".
In our example:
Let w = a b^p c^p d^p and mark the positions of the bs (of which there are p, so w satisfies the requirements of Ogden's Lemma), and let u,x,y,z,v be a decomposition satisfying the conditions from Ogden's lemma (z=uxyzv).
If x or z contain multiple symbols, then u x^2 y z^2 w is not in L, because there will be symbols in the wrong order (consider (bc)^2 = bcbc).
Either x or z must contain a b (by Lemma condition 1.)
This leaves us with five cases to check (for i,j>0):
x=epsilon, z=b^i
x=a, z=b^i
x=b^i, z=c^j
x=b^i, z=d^j
x=b^i, z=epsilon
in every case (by comparing the number of bs, cs and ds) we can see that u x^2 v y^2 z is not in L (and we have a contradiction (!) to the language being context-free, that is, we've proved that L is not context free).
.
To summarise, L is not context-free, but this cannot be demonstrated using The Pumping Lemma (but can by Ogden's Lemma) and thus we can say that:
Ogden's lemma is a second, stronger pumping lemma for context-free languages.
I'm not too sure about how to use Ogden's lemma here but your "proof" is wrong. When using the pumping lemma to prove that a language is not context free you cannot choose the splitting into uvxyz. The splitting is chosen "for you" and you have to show that the lemma is not fulfilled for any uvxyz.