GATE 2014 Question 35: Spotting Recognizable Languages
Here we go over a GATE exam problem about a language of Turing Machines that accept some string of length 2020. We then classify this language as being undecidable but recognizable. The proof of undecidability is supposing a decider for it exists, then obtaining a decider for A_TM from that, which we know cannot exist. The proof of recognizable comes from "brute force" parallel simulation of all strings of length 2020, and accepting if any such string is accepted.
▶ADDITIONAL QUESTIONS◀ 1. Could we have applied Rice's Theorem to this problem? 2. What can we say about the complement of L?
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▶ABOUT ME◀ I am a professor of Computer Science, and am passionate about CS theory. I have taught over 12 courses at Arizona State University, as well as Colgate University, including several sections of undergraduate theory. ... https://www.youtube.com/watch?v=YSgmw9t7OQs
Here we do two off-the-cuff examples of the NFA to DFA (Powerset) construction. The idea is to take the epsilon closure of the start state, and then repeatedly find "where to go" on each of the input symbols, always taking into account epsilon closure of the resulting states. Then, the final states are the ones that include an originally final state in the NFA. The second example is a slight modification of the first, and shows how small changes in the NFA can result in dramatically many more states.
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▶ADDITIONAL QUESTIONS◀
1. How large can the NFA be compared to the DFA?
2. What happened to all of the other subsets of the NFA's states that weren't considered here?
▶SEND ME THEORY QUESTIONS◀
ryan.e.dougherty@icloud.com
▶ABOUT ME◀
I am a professor of Computer Science, and am passionate about CS theory. I have taught over 12 courses at Arizona State University, as well as Colgate University, including several sections of undergraduate theory.
▶ABOUT THIS CHANNEL◀
The theory of computation is perhaps the fundamental theory of computer science. It sets out to define, mathematically, what exactly computation is, what is feasible to solve using a computer, and also what is not possible to solve using a computer. The main objective is to define a computer mathematically, without the reliance on real-world computers, hardware or software, or the plethora of programming languages we have in use today. The notion of a Turing machine serves this purpose and defines what we believe is the crux of all computable functions.
This channel is also about weaker forms of computation, concentrating on two classes: regular languages and context-free languages. These two models help understand what we can do with restricted means of computation, and offer a rich theory using which you can hone your mathematical skills in reasoning with simple machines and
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https://www.youtube.com/watch?v=oCRbwLXvNjI
Here we show that the language of all strings of the form 0^n 1^m where n is strictly less than 3m is not regular. This is a standard pumping lemma proof, other than picking the right string at the beginning!
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I am a professor of Computer Science, and am passionate about CS theory. I have taught many courses at several different universities, including several sections of undergraduate and graduate theory-level classes.
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https://www.youtube.com/watch?v=N3G2CX2p_po
Here we show that the strings of the form 0^2^n (i.e., all powers of 2) is not regular. This mimics the proof that perfect squares is not regular (https://www.youtube.com/watch?v=Au8vH07IXuM&ab_channel=EasyTheory).
What is the pumping lemma for regular languages? It is a way to help prove that certain languages are not regular (i.e., those languages accepted by a DFA). If a language is regular, then it has certain properties; on the other hand, if it does not have those properties, then it cannot be regular. See https://www.youtube.com/watch?v=x2J5kaf6gjg&ab_channel=EasyTheory for more details.
Thanks to the following supporters of the channel for helping support this video. If you want to contribute, links are below. Names are listed in alphabetical order by surname.
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▶SEND ME THEORY QUESTIONS◀
ryan.e.dougherty@icloud.com
▶ABOUT ME◀
I am a professor of Computer Science, and am passionate about CS theory. I have taught many courses at several different universities, including several sections of undergraduate and graduate theory-level classes.
...
https://www.youtube.com/watch?v=n6VYcrbz7Y8
Easy Theory Website: https://www.easytheory.org
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If you like this content, please consider subscribing to my channel: https://www.youtube.com/channel/UC3VY6RTXegnoSD_q446oBdg?sub_confirmation=1
▶ABOUT ME◀
I am a professor of Computer Science, and am passionate about it. I have taught many courses at several different universities, including several sections of undergraduate and graduate theory-level classes.
The views expressed in this video are not reflective of any of my current or former employers.
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https://www.youtube.com/watch?v=KvqW6SKr_XQ
Here we give some problems on regular grammars, that deal with the conversions between them and NFAs and DFAs, such as how big the regular grammar and/or NFA/DFA will be, and is there a direct conversion between a regular grammar and DFA (without using an intermediate NFA)?
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ryan.e.dougherty@icloud.com
▶ABOUT ME◀
I am a professor of Computer Science, and am passionate about CS theory. I have taught many courses at several different universities, including several sections of undergraduate and graduate theory-level classes.
...
https://www.youtube.com/watch?v=EH0JZp0D3UI
Here we give a proof of the "master" theorem, which is a derivation of the runtime for all recursive algorithms obeying a recurrence of the form T(n) = aT(n/b) + f(n) (i.e., a subproblems of size n/b, and f(n) time to combine the solutions). We see that there are 3 different cases, mainly based on how big f(n) is compared to a, b, and n. We look at a "computation tree" to show what the runtime across all nodes will be.
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▶SEND ME THEORY QUESTIONS◀
ryan.e.dougherty@icloud.com
▶ABOUT ME◀
I am a professor of Computer Science, and am passionate about CS theory. I have taught many courses at several different universities, including several sections of undergraduate and graduate theory-level classes.
...
https://www.youtube.com/watch?v=ZGmUTe1uiE0
Here we give two problems related to context-free grammars and context-free languages.
What is a context-free grammar? It is a set of 4 items: a set of "variables," a set of "terminals," a "start variable," and a set of rules. Each rule must involve a single variable on its "left side", and any combination of variables and terminals on its right side. See https://www.youtube.com/watch?v=h1OSmLSacNA&ab_channel=EasyTheory for more details.
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▶SEND ME THEORY QUESTIONS◀
ryan.e.dougherty@icloud.com
▶ABOUT ME◀
I am a professor of Computer Science, and am passionate about CS theory. I have taught many courses at several different universities, including several sections of undergraduate and graduate theory-level classes.
...
https://www.youtube.com/watch?v=COTzXfoaGpM
A lot of students and inexperienced programmers will loop over a list by index in Python, when in reality they should be looping over it by value. Even if an index is needed (which is a small minority of the time), then one should use enumerate because it gives the index and value at the same time. Further, looping by index does not always work with non-lists, such as sets or dictionaries.
Thanks to the following supporters of the channel for helping support this video. If you want to contribute, links are below.
Dolev Abuhazira, Josh Hibschman, Micah Wood, Morgan Jones, Patrik Keinonen, Simone Glinz, Tao Su, Timothy Gorden, unit220, Valentine Eben
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▶SEND ME THEORY QUESTIONS◀
ryan.e.dougherty@icloud.com
▶ABOUT ME◀
I am a professor of Computer Science, and am passionate about CS theory. I have taught many courses at several different universities, including several sections of undergraduate and graduate theory-level classes.
...
https://www.youtube.com/watch?v=vvJjN8MqErE
Here we give a help session on converting from any nondeterministic finite automaton (NFA) to an equivalent deterministic one (DFA). This was recorded on 1 February 2017.
Timeline:
0:00 - Intro
2:00 - NFA Definition
4:00 - NFA to DFA Powerset Construction
22:00 - NFA Construction Example
29:50 - Closure Under Star Example
34:20 - NFA to DFA Example 1
41:15 - NFA to DFA Example 2
Thanks to the following supporters of the channel for helping support this video. If you want to contribute, links are below. Names are listed in alphabetical order by surname.
Platinum: Micah Wood
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▶SEND ME THEORY QUESTIONS◀
ryan.e.dougherty@icloud.com
▶ABOUT ME◀
I am a professor of Computer Science, and am passionate about CS theory. I have taught many courses at several different universities, including several sections of undergraduate and graduate theory-level classes.
...
https://www.youtube.com/watch?v=dRKGZWAuxGU