## Regular expression for strings not starting with 10

How can I construct a regular expression for the language over $$\{0,1\}$$ which is the complement of the language represented by the regular expression $$10(0+1)^*$$?

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## Regular expressions for set of all strings on alphabet $\{a, b\}$

I came across following regular expressions which equals $$(a+b)^*$$ (set of all strings on alphabet $$\{a, b\}$$):

• $$(a^*+bb^*)^*$$
• $$(a^*b+b^*a)^*$$
• $$(a^*bb^*+b^*ab^*)^*(a^*b+b^*a)^*b^*a^*$$

I want to generalise different ways in which we can append to original regular expression $$(a+b)^*$$, to not to change its meaning and still get set of all strings on alphabet $$\{a, b\}$$. I think we can do this in two ways :

• P1: We can concatenate anything to $$a$$ and $$b$$ inside brackets of $$(a+b)^*$$
• P2: We can concatenate $$(a+b)^*$$ with any regular expression which has star at most outer level ($$(…)^*$$)

• P3: I know $$(a+b)^* = (a^*+b)^* = (a+b^*)^*= (a^*+b^*)^*$$. So I guess P1 and P2 also applies to them.

Am I correct with P’s?

Q. Also I know $$(a+b)^*=(a^*b^*)^*=b^*(a^*b)^*=(ab^*)^*a^*$$. Can we append some pattern of regular expressions to these also to not to change their original meaning?

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## Evaluating predicate on binary strings

Consider two unknown binary strings $$X = x_{1} x_{2} \dots x_{n^{2}}, \quad Y = y_{1} y_{2} \dots y_{n^{2}}, \quad x_{i}, y_{i} \in \{0, 1\} .$$ We may request a string $$Z = z_{1} z_{2} \dots z_{n^{2}}$$, where $$z_{i} = x_{i}$$ or $$z_{i} = y_{i}$$, no more than $$n + 1$$ times. So, for every request we set required $$z_{i}$$ (that is, $$x_{i}$$ or $$y_{i}$$) for every $$i$$.

Moreover, we have $$n$$ bits of unwritable memory, namely, every bit of that memory is set once and then does not change. This memory is avaliable all the time, but requested $$Z$$ strings drop out before the next request, so, we don’t have complete list of all requested $$Z$$ strings.

The problem is to check if $$X = Y$$ with given $$n$$ bits and $$n + 1$$ times for $$Z$$ string request.

There is an extra question: is it possible to use $$\mathcal{O}(\log^{2}(n))$$ bits of memory and $$\mathcal{O}(\log(r))$$ requests.

I don’t really understand area of CS that is closely related to the problem, could anyone give a hint?

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## How many “compressible” strings are there?

Let’s say that a string of length $$N$$ is “compressible” iff its Kolmogorov complexity is less than $$N$$. To keep it simple, we can assume binary strings for this.

It is easy to see that almost all binary strings of length $$N$$ are incompressible by using the pigeonhole principle.

So my question is, how many strings of length $$N$$ are compressible?

In particular, let’s assume that $$K(S)$$ is the Kolmogorov complexity of binary string $$S$$, which is of length $$N$$. Then I have the following three questions:

1. Of the $$2^N$$ binary strings $$S$$ of length $$N$$, how many have $$K(S) < N$$?
2. Of the $$2^N$$ binary strings $$S$$ of length $$N$$, how many have $$K(S) \leq N/2$$?
3. Of the $$2^N$$ binary strings $$S$$ of length $$N$$, how many have $$K(S) \leq \log N$$?

All of the above are for sufficiently large $$N$$.

## Regular Expressions to accept strings where every two successive 0s are followed by a 1

How do I define a Regular Expression for the language L = {w|every two successive 0s are followed by a 1} ?

I don’t have any idea on how to even approach this problem, primarily because it says “every two 0s“.

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## Build a DFA that accepts strings over {0,1,2} that are divided by 3 and doesn’t include the substring 012

Ive tried to draw it but im not finding the right way. Ive also read it “How to prove a language is regular?” and here – https://math.stackexchange.com/questions/1556655/build-a-dfa-that-accepts-strings-over-0-1-2-that-are-divided-by-3-and-do , but still in the same situation. hope someone can help or give a lead.

Thats what ive got so far: Posted on Categories proxies

## Why is the lower bound for sorting strings Ω(d + nlogn)?

I’m taking an Advanced Algorithms course. I’m currently studying efficient algorithms for sorting strings. In this chapter, it is provided a lower bound for the time complexity of Ω(d + nlogn), where d is the sum of the distinguishing prefixes of all the strings in our set S and n is the cardinality of the strings set S. The book says this is the minimum number of comparisons any algorithm must take, and I cannot figure out why. Can you help me? Thank you.

## Finding the number of distinct strings in regular expression

Given the regular expression $$(1 + \varepsilon + 0 )(1 + \varepsilon + 0 )(1 + \varepsilon + 0 )(1 + \varepsilon + 0 )$$, how many distinct strings are in the language? How do you determine this from looking at the regular expression? Do I have to generate a table of all possible combinations or is there a more straightforward way?

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## Decrypting strings C and D without knowing the keys

I have a question:

Let’s assume I encrypt string A with string B(key) (the result is string C) and then I encrypt string B with string A(key) (the result is string D). Let’s say using AES algorithm.

If an attacker knows that AES is the algorithm I used and also has C and D, can they figure out strings A and B?

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## Regular expression for set of strings with no two consecutive 1’s?

I’m having trouble figuring out a regular expression over the alphabet {0,1} that contains all strings with no TWO consecutive 1’s.

I’m also wondering if there is a pattern that could be extended to three consecutive 1s. etc…

Would something like this be correct?

(0 U (10))* U (0 U (10))* 1

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