l07 - 6.896 Sublinear Time Algorithms February 27, 2007...

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Unformatted text preview: 6.896 Sublinear Time Algorithms February 27, 2007 Lecture 7 Lecturer: Ronitt Rubinfeld Scribe: Brendan Juba 1 R e c a p Recall from last time that we were considering the boolean hypercube, { 1 } n . We had defined the partial ordering, for x, y { 1 } n , x y if i x i y i . We said that a probability distribution p was monotone over { 1 } n if x y p x p y . Recall also that wed defined bias ( X ) = i X i . Additive Chernoff Bound (Hoeffding) For X 1 , . . . , X m i.i.d. random variables with range [ a, a ], = 1 m m i =1 X i , and = E [ ], > Pr[ | | > ] 2 exp 2 2 a 2 m . We were in the middle of the analysis of the following algorithm: Algorithm: Test Uniform 1. Pick s = ( n 2 log n ) samples X (1) , . . . , X ( s ) p . 2. If any X ( i ) has | bias ( X ( i ) ) | > p 2 n log(20 s ), stop and output nonuniform. 3. Let = 1 s s i =1 bias ( X ( i ) ). 4. If 4 , output uniform. Otherwise, output nonuniform. 2 Analysis of Uniformity Test Last time, we saw the analysis of the case where p = U D . Furthermore, in the case where | p U D | > , we proved the following claim: Claim 1 If | p U D | > , then E p [ bias ( X )] . Today we will complete the analysis of this second case, which is broken into two subcases. We wish to show that in both subcases, the algorithm outputs nonuniform with probability at least 2 / 3. Case 2a: Pr p [ | bias ( X ) | > p 2 n log(20 s )] 10 s Notice that on any single sample X ( i ) , Pr[step 2 does not output nonuniform on X ( i ) ] 1 10 s where, since we have s independent samples, Pr[step 2 does not output nonuniform] 1 10 s s < 1 3 (note that this probability is actually quite small, but 1/3 is sucient) 1-1 -1 . .. -1 D D 1 1 . .. 1 Figure 1 : The set D : D without the tails of the distribution. Case 2b: Pr p [ | bias ( X ) | > p 2 n log(20 s )] < 10 s In this case, we might stop in step 2 (and if we do, thats great for us), but it will suce to show that step 4 will output nonuniform with suciently high probability otherwise. Notice that if step 2 passes, each i satisfies | bias ( X ( i ) ) | p 2 n log(20 s ), so our samples are from a distribution that has been conditioned on this event: letting D = { 1 } n be our sample space, the event we are concerned with is D = { x D : | bias ( X ) | p 2 n log(20 s ) } . If we take p = p | D , then after step 2, we know we have a sample drawn from p . Of course, since passing step 2 suggests that the probability of obtaining a sample from D \ D is very small, we anticipate that p is close to p . We will make this precise by bounding E X p [ | bias ( X ) | ] E X p [ | bias ( X ) | ]: Claim 2 In this case, for suciently large n , E X p [ | bias ( X ) | ] E X p [ | bias ( X ) | ] / 8 Proof It is easy to see that E X p [ | bias...
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This note was uploaded on 04/02/2010 for the course CS 6.896 taught by Professor Ronittrubinfeld during the Fall '04 term at MIT.

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l07 - 6.896 Sublinear Time Algorithms February 27, 2007...

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