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	<title>Comments for Windows On Theory</title>
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	<link>http://windowsontheory.org</link>
	<description>A Research Blog</description>
	<lastBuildDate>Fri, 17 May 2013 13:32:25 +0000</lastBuildDate>
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		<title>Comment on Reasons to care: In honor of Scott Aaronson by Boaz Barak</title>
		<link>http://windowsontheory.org/2013/05/06/reasons-to-care-in-honor-of-scott-aaronson/#comment-6398</link>
		<dc:creator><![CDATA[Boaz Barak]]></dc:creator>
		<pubDate>Fri, 17 May 2013 13:32:25 +0000</pubDate>
		<guid isPermaLink="false">http://windowsontheory.org/?p=2376#comment-6398</guid>
		<description><![CDATA[Hi Gil,
If you simply know that the circuit exists it doesn&#039;t help you much, since you can always assume it exists and see what happens.
If you are given black-box access to the circuit, then it&#039;s a well known problem of learning a circuit with oracle queries. This is widely believed to be computationally intractable, even if you make the necessary relaxation that you only need to find a circuit that approximates your black-box. If cryptographic pseudorandom functions exist, they are a great example of circuits that you cannot learn even very weakly from oracle queries.
There is a paper by Bshouty Cleve Gavalda Kannan and Tamon showing you can learn such circuits with access to NP oracle or oracles higher in the polynomial hierarchy (depending on the exact form of queries allowed), see http://www.cs.technion.ac.il/~bshouty/MyPapers/Oracles%20and%20Queries%20that%20are%20Sufficient%20for%20Exact%20Learning/bshouty96oracles.pdf]]></description>
		<content:encoded><![CDATA[<p>Hi Gil,<br />
If you simply know that the circuit exists it doesn&#8217;t help you much, since you can always assume it exists and see what happens.<br />
If you are given black-box access to the circuit, then it&#8217;s a well known problem of learning a circuit with oracle queries. This is widely believed to be computationally intractable, even if you make the necessary relaxation that you only need to find a circuit that approximates your black-box. If cryptographic pseudorandom functions exist, they are a great example of circuits that you cannot learn even very weakly from oracle queries.<br />
There is a paper by Bshouty Cleve Gavalda Kannan and Tamon showing you can learn such circuits with access to NP oracle or oracles higher in the polynomial hierarchy (depending on the exact form of queries allowed), see <a href="http://www.cs.technion.ac.il/~bshouty/MyPapers/Oracles%20and%20Queries%20that%20are%20Sufficient%20for%20Exact%20Learning/bshouty96oracles.pdf" rel="nofollow">http://www.cs.technion.ac.il/~bshouty/MyPapers/Oracles%20and%20Queries%20that%20are%20Sufficient%20for%20Exact%20Learning/bshouty96oracles.pdf</a></p>
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		<title>Comment on Reasons to care: In honor of Scott Aaronson by Gil Kalai</title>
		<link>http://windowsontheory.org/2013/05/06/reasons-to-care-in-honor-of-scott-aaronson/#comment-6392</link>
		<dc:creator><![CDATA[Gil Kalai]]></dc:creator>
		<pubDate>Fri, 17 May 2013 08:35:31 +0000</pubDate>
		<guid isPermaLink="false">http://windowsontheory.org/?p=2376#comment-6392</guid>
		<description><![CDATA[Hi Boaz, actually I think that the point that in certain cases of natural algorithms P/poly is better model than P is quite interesting.

I have a related question that might be quite silly but let me ask anyway. Suppose that you have a decision problem in NP and you are guaranteed that the problem is in P/poly or specifically that it can be solved by a circuit of size $latex 5n^3$ when the input is of size n. The question is how hard it is to find a polynomial size circuit that solves the problem. (In its general case.) Is it, or can it be computationally intractable? Can it be NP-hard or something like that?]]></description>
		<content:encoded><![CDATA[<p>Hi Boaz, actually I think that the point that in certain cases of natural algorithms P/poly is better model than P is quite interesting.</p>
<p>I have a related question that might be quite silly but let me ask anyway. Suppose that you have a decision problem in NP and you are guaranteed that the problem is in P/poly or specifically that it can be solved by a circuit of size <img src='http://s0.wp.com/latex.php?latex=5n%5E3&amp;bg=ffffff&amp;fg=333333&amp;s=0' alt='5n^3' title='5n^3' class='latex' /> when the input is of size n. The question is how hard it is to find a polynomial size circuit that solves the problem. (In its general case.) Is it, or can it be computationally intractable? Can it be NP-hard or something like that?</p>
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		<title>Comment on On Endre Szemer&#233;di&#8217;s Gifts to Computer Science by On the importance of the alphabet &#124; Windows On Theory</title>
		<link>http://windowsontheory.org/2012/03/29/on-endre-szemerdis-gifts-to-computer-science/#comment-6349</link>
		<dc:creator><![CDATA[On the importance of the alphabet &#124; Windows On Theory]]></dc:creator>
		<pubDate>Tue, 14 May 2013 04:51:43 +0000</pubDate>
		<guid isPermaLink="false">http://windowsontheory.org/?p=645#comment-6349</guid>
		<description><![CDATA[[&#8230;] Optimal codes of distance d: Arguably the mother of all such problems. How many parity checks must a ary length  code of distance  have? Here we think of  as a constant and  going to infinity. For  the Hamming bound tells us that  parity checks are necessary. The BCH construction tells us that this is also sufficient. For larger alphabets, the best lower bound is still the Hamming bound, and the Hamming bound is still . Alas the BCH upper bound is now . For more on this problem and its connections to additive combinatorics, see here. [&#8230;]]]></description>
		<content:encoded><![CDATA[<p>[&#8230;] Optimal codes of distance d: Arguably the mother of all such problems. How many parity checks must a ary length  code of distance  have? Here we think of  as a constant and  going to infinity. For  the Hamming bound tells us that  parity checks are necessary. The BCH construction tells us that this is also sufficient. For larger alphabets, the best lower bound is still the Hamming bound, and the Hamming bound is still . Alas the BCH upper bound is now . For more on this problem and its connections to additive combinatorics, see here. [&#8230;]</p>
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		<title>Comment on Learning Juntas by On the importance of the alphabet &#124; Windows On Theory</title>
		<link>http://windowsontheory.org/2012/11/27/755/#comment-6348</link>
		<dc:creator><![CDATA[On the importance of the alphabet &#124; Windows On Theory]]></dc:creator>
		<pubDate>Tue, 14 May 2013 04:51:40 +0000</pubDate>
		<guid isPermaLink="false">http://windowsontheory.org/?p=755#comment-6348</guid>
		<description><![CDATA[[&#8230;] last post, we saw that the problem of learning juntas, hard as it is over Boolean inputs, seems even worse [&#8230;]]]></description>
		<content:encoded><![CDATA[<p>[&#8230;] last post, we saw that the problem of learning juntas, hard as it is over Boolean inputs, seems even worse [&#8230;]</p>
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		<title>Comment on Reasons to care: In honor of Scott Aaronson by Gil Kalai</title>
		<link>http://windowsontheory.org/2013/05/06/reasons-to-care-in-honor-of-scott-aaronson/#comment-6336</link>
		<dc:creator><![CDATA[Gil Kalai]]></dc:creator>
		<pubDate>Mon, 13 May 2013 05:33:09 +0000</pubDate>
		<guid isPermaLink="false">http://windowsontheory.org/?p=2376#comment-6336</guid>
		<description><![CDATA[Boaz, I think we are in agreement that the proof of Poincare conjecture required creativity. Connecting creativity to computational complexity intractability in this context is artificial. Why do you think that the third statement below makes more sense than the first two? 

1) The general, problem of finding a proof to a mathematical statement can be undecidable, and therefore the proof of Poncare conjecture reflects creativity

2) The size of a  proof of a general mathematical statement that has a proof can be huge in terms of the size of the statement. Therefore the proof of Poincare conjecture reflects creativity.

3) Finding a proof of a mathematical statement can be computationally hard even if there is a small length proof. Therefore the proof of Poincare conjecture reflects creativity.]]></description>
		<content:encoded><![CDATA[<p>Boaz, I think we are in agreement that the proof of Poincare conjecture required creativity. Connecting creativity to computational complexity intractability in this context is artificial. Why do you think that the third statement below makes more sense than the first two? </p>
<p>1) The general, problem of finding a proof to a mathematical statement can be undecidable, and therefore the proof of Poncare conjecture reflects creativity</p>
<p>2) The size of a  proof of a general mathematical statement that has a proof can be huge in terms of the size of the statement. Therefore the proof of Poincare conjecture reflects creativity.</p>
<p>3) Finding a proof of a mathematical statement can be computationally hard even if there is a small length proof. Therefore the proof of Poincare conjecture reflects creativity.</p>
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		<title>Comment on Reasons to care: In honor of Scott Aaronson by Boaz Barak</title>
		<link>http://windowsontheory.org/2013/05/06/reasons-to-care-in-honor-of-scott-aaronson/#comment-6333</link>
		<dc:creator><![CDATA[Boaz Barak]]></dc:creator>
		<pubDate>Mon, 13 May 2013 01:54:25 +0000</pubDate>
		<guid isPermaLink="false">http://windowsontheory.org/?p=2376#comment-6333</guid>
		<description><![CDATA[&lt;i&gt; If you need to recover the (polynomial) algorithm performed by an unknown computer chip and need to go over all the possible chips of the same size, this is intractable (yet constant), and you may have similar issues for algorithms for natural processes.&lt;/i&gt;

This is an argument why in certain cases P/poly is a better model for &quot;efficient computation&quot; than P. Indeed typically in cryptography, we would like our algorithms to be in P (i.e., explicit algorithm with no large parameters) but want them to be secure even against adversaries in P/poly (even against natural or artificial processes that have large parameters.

&lt;i&gt; Whether you are creative or not, there are no systematic ways to solve intractable questions, or any good chance to stumble upon the solution.&lt;/i&gt;

Maybe the issue here is my complexity/cryptogaphy background. I think of an &quot;intractable problem&quot; as a question of mapping inputs to outputs (which in the case of NP have some efficiently verifiable relation between them). The problem is intractable because there is no efficient way that *always* (or in the case of average-case complexity, with good probability) succeeds in mapping an input to the correct output. But even for intractable problems, you may get lucky and manage to find the right output for some input.

You can say that maybe each input defines its own particular problem, and some inputs are intractable and others are easy, but this is a bit problematic, partly because for every particular input there always exists a small circuit that finds the right output (i.e., a circuit with this output hardwired into it), and, as you argue above, it&#039;s not clear if we shouldn&#039;t allow such circuits when we model &quot;efficient computation&quot;.

So, while finding proofs for mathematical statements is hard in general, people still succeed in doing so for particular statements, and when they do so, we feel they are sometimes creative, perhaps exactly because we do not know of any systematic &quot;cookbook&quot; way to find such proofs.]]></description>
		<content:encoded><![CDATA[<p><i> If you need to recover the (polynomial) algorithm performed by an unknown computer chip and need to go over all the possible chips of the same size, this is intractable (yet constant), and you may have similar issues for algorithms for natural processes.</i></p>
<p>This is an argument why in certain cases P/poly is a better model for &#8220;efficient computation&#8221; than P. Indeed typically in cryptography, we would like our algorithms to be in P (i.e., explicit algorithm with no large parameters) but want them to be secure even against adversaries in P/poly (even against natural or artificial processes that have large parameters.</p>
<p><i> Whether you are creative or not, there are no systematic ways to solve intractable questions, or any good chance to stumble upon the solution.</i></p>
<p>Maybe the issue here is my complexity/cryptogaphy background. I think of an &#8220;intractable problem&#8221; as a question of mapping inputs to outputs (which in the case of NP have some efficiently verifiable relation between them). The problem is intractable because there is no efficient way that *always* (or in the case of average-case complexity, with good probability) succeeds in mapping an input to the correct output. But even for intractable problems, you may get lucky and manage to find the right output for some input.</p>
<p>You can say that maybe each input defines its own particular problem, and some inputs are intractable and others are easy, but this is a bit problematic, partly because for every particular input there always exists a small circuit that finds the right output (i.e., a circuit with this output hardwired into it), and, as you argue above, it&#8217;s not clear if we shouldn&#8217;t allow such circuits when we model &#8220;efficient computation&#8221;.</p>
<p>So, while finding proofs for mathematical statements is hard in general, people still succeed in doing so for particular statements, and when they do so, we feel they are sometimes creative, perhaps exactly because we do not know of any systematic &#8220;cookbook&#8221; way to find such proofs.</p>
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		<title>Comment on Reasons to care: In honor of Scott Aaronson by Gil Kalai</title>
		<link>http://windowsontheory.org/2013/05/06/reasons-to-care-in-honor-of-scott-aaronson/#comment-6322</link>
		<dc:creator><![CDATA[Gil Kalai]]></dc:creator>
		<pubDate>Sat, 11 May 2013 17:48:27 +0000</pubDate>
		<guid isPermaLink="false">http://windowsontheory.org/?p=2376#comment-6322</guid>
		<description><![CDATA[Dear Boaz, I think we are in agreement in strongly believing that NP is different than P, and that finding efficient algorithm for NP-hard problems will have amazing consequences. And also that this is a very interesting and important question. 

&quot;I don’t think creative people have a systematic way to solve intractable questions, but in some cases (like the proof of the Poincare conjecture), they manage to stumble upon the solution.&quot;

Here we disagree. Whether you are creative or not, there are no systematic ways to solve intractable questions, or any good chance to stumble upon the solution. (This is why they referred to as intractable.)  The Poincare conjecture is simply not intractable. Creativity is related to the phenomenon that verifying is easier than finding, but it is not related to intractability.

We can have some ideas about mathematical questions that are indeed probably intractable, like computing the Ramsey number R(50,50) or finding the length of the shorter proof of the prime number theorem in a prescribed proof system. And indeed, these tasks will not be achieved  systematically or by chance, by creative people or by other people or by computers.

&quot;Obviously if the output we try to compute is not determined by the input then no computation can help.&quot;

Yes, but this is a huge issue in the larger area of computation, related to modeling, numerical analysis, chaotic behavior etc. We should care about it too!

&quot;For P we assume that the algorithm has a absolute constant..&quot;

That&#039;s correct, but when we apply insights from cc to real-life problems we obtain distinction between huge intractable constants and constants within reach. If you need to recover the (polynomial) algorithm performed by an unknown computer chip and need to go over all the possible chips of the same size, this is intractable (yet constant), and you may have similar issues for algorithms for natural processes.

&quot;I believe that by the time we get a proof, this conjectural picture will only be a small part of the new things we’ll learn about computation, but this of course purely speculative.&quot;

This is certainly a terrific possibility!]]></description>
		<content:encoded><![CDATA[<p>Dear Boaz, I think we are in agreement in strongly believing that NP is different than P, and that finding efficient algorithm for NP-hard problems will have amazing consequences. And also that this is a very interesting and important question. </p>
<p>&#8220;I don’t think creative people have a systematic way to solve intractable questions, but in some cases (like the proof of the Poincare conjecture), they manage to stumble upon the solution.&#8221;</p>
<p>Here we disagree. Whether you are creative or not, there are no systematic ways to solve intractable questions, or any good chance to stumble upon the solution. (This is why they referred to as intractable.)  The Poincare conjecture is simply not intractable. Creativity is related to the phenomenon that verifying is easier than finding, but it is not related to intractability.</p>
<p>We can have some ideas about mathematical questions that are indeed probably intractable, like computing the Ramsey number R(50,50) or finding the length of the shorter proof of the prime number theorem in a prescribed proof system. And indeed, these tasks will not be achieved  systematically or by chance, by creative people or by other people or by computers.</p>
<p>&#8220;Obviously if the output we try to compute is not determined by the input then no computation can help.&#8221;</p>
<p>Yes, but this is a huge issue in the larger area of computation, related to modeling, numerical analysis, chaotic behavior etc. We should care about it too!</p>
<p>&#8220;For P we assume that the algorithm has a absolute constant..&#8221;</p>
<p>That&#8217;s correct, but when we apply insights from cc to real-life problems we obtain distinction between huge intractable constants and constants within reach. If you need to recover the (polynomial) algorithm performed by an unknown computer chip and need to go over all the possible chips of the same size, this is intractable (yet constant), and you may have similar issues for algorithms for natural processes.</p>
<p>&#8220;I believe that by the time we get a proof, this conjectural picture will only be a small part of the new things we’ll learn about computation, but this of course purely speculative.&#8221;</p>
<p>This is certainly a terrific possibility!</p>
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		<title>Comment on Reasons to care: In honor of Scott Aaronson by Ryan O'Donnell</title>
		<link>http://windowsontheory.org/2013/05/06/reasons-to-care-in-honor-of-scott-aaronson/#comment-6310</link>
		<dc:creator><![CDATA[Ryan O'Donnell]]></dc:creator>
		<pubDate>Sat, 11 May 2013 01:00:19 +0000</pubDate>
		<guid isPermaLink="false">http://windowsontheory.org/?p=2376#comment-6310</guid>
		<description><![CDATA[This is reasonable response.  I definitely agree with you that people need to cut us some slack when it comes to &quot;evidence in favor of P \neq NP&quot;.  Given that we cannot prove anything close to the theorem (or its negation), one ought to be a bit lenient when it comes to accepting evidence.

OTOH, I also like Anıl&#039;s comment below.]]></description>
		<content:encoded><![CDATA[<p>This is reasonable response.  I definitely agree with you that people need to cut us some slack when it comes to &#8220;evidence in favor of P \neq NP&#8221;.  Given that we cannot prove anything close to the theorem (or its negation), one ought to be a bit lenient when it comes to accepting evidence.</p>
<p>OTOH, I also like Anıl&#8217;s comment below.</p>
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		<title>Comment on Reasons to care: In honor of Scott Aaronson by Boaz Barak</title>
		<link>http://windowsontheory.org/2013/05/06/reasons-to-care-in-honor-of-scott-aaronson/#comment-6307</link>
		<dc:creator><![CDATA[Boaz Barak]]></dc:creator>
		<pubDate>Fri, 10 May 2013 19:57:07 +0000</pubDate>
		<guid isPermaLink="false">http://windowsontheory.org/?p=2376#comment-6307</guid>
		<description><![CDATA[This appears in several textbooks and lecture notes (including my own), see for example third slide here http://cosec.bit.uni-bonn.de/fileadmin/user_upload/teaching/07ss/cryptabit/Cryptabit06/LcCharles_Tue.pdf
 (note that there different variants of pseudorandom genrators, but am talking here about the variant used for cryptography, others need only milder assumptions such as P different from EXP).]]></description>
		<content:encoded><![CDATA[<p>This appears in several textbooks and lecture notes (including my own), see for example third slide here <a href="http://cosec.bit.uni-bonn.de/fileadmin/user_upload/teaching/07ss/cryptabit/Cryptabit06/LcCharles_Tue.pdf" rel="nofollow">http://cosec.bit.uni-bonn.de/fileadmin/user_upload/teaching/07ss/cryptabit/Cryptabit06/LcCharles_Tue.pdf</a><br />
 (note that there different variants of pseudorandom genrators, but am talking here about the variant used for cryptography, others need only milder assumptions such as P different from EXP).</p>
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		<title>Comment on Reasons to care: In honor of Scott Aaronson by Boaz Barak</title>
		<link>http://windowsontheory.org/2013/05/06/reasons-to-care-in-honor-of-scott-aaronson/#comment-6306</link>
		<dc:creator><![CDATA[Boaz Barak]]></dc:creator>
		<pubDate>Fri, 10 May 2013 19:49:23 +0000</pubDate>
		<guid isPermaLink="false">http://windowsontheory.org/?p=2376#comment-6306</guid>
		<description><![CDATA[&lt;i&gt;Perhaps, this vast conjectural picture and theory around it are even more important that a proof will be but this is yet to be seen.&lt;/i&gt;

I believe that by the time we get a proof, this conjectural picture will only be a small part of the new things we&#039;ll learn about computation, but this of course purely speculative.

&lt;i&gt;For example, if the P-algorithm depends on a secret that if we don’t know we need exponential search to find.&lt;/i&gt;

This seems to apply more to non-uniform computation, which is the class P/poly. For P we assume that the algorithm has a absolute constant description size that does not depend on the length of the input.

&lt;i&gt; Our inability today to give an accurate weather forecast for May 9 2014 is not a computational complexity issue. &lt;/i&gt;

Obviously if the output we try to compute is not determined by the input then no computation can help. I don&#039;t know if that&#039;s the case or not for weather prediction. As I commented above in the discussion with Ryan, though I am not an expert on machine learning, I believe an algorithm for SAT will certainly help in many prediction tasks. In particular, I think sometimes the problem is finding which features in the data are relevant for the prediction, something that you could do if you had a SAT algorithm.

&lt;i&gt;There is no reason to think that any aspect of human creativity involves solving computational intractable questions&lt;/i&gt;

I don&#039;t think creative people have a systematic way to solve intractable questions, but in some cases (like the proof of the Poincare conjecture), they manage to stumble upon the solution. I agree that computers could also do the same even without P=NP, but would be &lt;i&gt;much&lt;/i&gt; better at it with an efficient SAT algorithm.]]></description>
		<content:encoded><![CDATA[<p><i>Perhaps, this vast conjectural picture and theory around it are even more important that a proof will be but this is yet to be seen.</i></p>
<p>I believe that by the time we get a proof, this conjectural picture will only be a small part of the new things we&#8217;ll learn about computation, but this of course purely speculative.</p>
<p><i>For example, if the P-algorithm depends on a secret that if we don’t know we need exponential search to find.</i></p>
<p>This seems to apply more to non-uniform computation, which is the class P/poly. For P we assume that the algorithm has a absolute constant description size that does not depend on the length of the input.</p>
<p><i> Our inability today to give an accurate weather forecast for May 9 2014 is not a computational complexity issue. </i></p>
<p>Obviously if the output we try to compute is not determined by the input then no computation can help. I don&#8217;t know if that&#8217;s the case or not for weather prediction. As I commented above in the discussion with Ryan, though I am not an expert on machine learning, I believe an algorithm for SAT will certainly help in many prediction tasks. In particular, I think sometimes the problem is finding which features in the data are relevant for the prediction, something that you could do if you had a SAT algorithm.</p>
<p><i>There is no reason to think that any aspect of human creativity involves solving computational intractable questions</i></p>
<p>I don&#8217;t think creative people have a systematic way to solve intractable questions, but in some cases (like the proof of the Poincare conjecture), they manage to stumble upon the solution. I agree that computers could also do the same even without P=NP, but would be <i>much</i> better at it with an efficient SAT algorithm.</p>
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