A strong direct product theorem in terms of the smooth rectangle bound

Transcription

1 A strong direct product theorem in terms of the smooth rectangle bound Rahul Jain Centre for Quantum Technologies and Department of Computer Science National U. Singapore Penghui Yao Centre for Quantum Technologies National U. Singapore Abstract A strong direct product theorem states that, in order to solve k instances of a problem, if we provide less than k times the resource required to compute one instance, then the probability of overall success is exponentially small in k. In this paper, we consider the model of two-way public-coin communication complexity and show a strong direct product theorem for all relations in terms of the smooth rectangle bound, introduced by Jain and Klauck [6] as a generic lower bound method in this model. Our result therefore implies a strong direct product theorem for all relations for which an asymptotically) optimal lower bound can be provided using the smooth rectangle bound. In fact we are not aware of any relation for which it is known that the smooth rectangle bound does not provide an optimal lower bound. This lower bound subsumes many of the other known lower bound methods, for example the rectangle bound a.k.a the corruption bound) [3], the smooth discrepancy bound a.k.a the γ 2 bound [28] which in turn subsumes the discrepancy bound), the subdistribution bound [7] and the conditional min-entropy bound [4]. As a consequence, our result reproves some of the known strong direct product results, for example for the Inner Product [26] function and the Set-Disjointness [25, 4] function. Recently smooth rectangle bound has been used to provide new tight lower bounds for several functions, for example for the Gap-Hamming Distance [8, 33] partial function, the Greater-Than function [35] and the Tribes [2] function. These results, along with our result, imply strong direct product for these functions. Smooth rectangle bound has also been used to provide near optimal lower bounds for several important functions and relations used to show exponential separations between classical and quantum communication complexity for example by Raz [30], Gavinsky [] and Klartag and Regev [32]. These results combined with our result imply near optimal strong direct product results for these functions and relations. We show our result using information theoretic arguments. A key tool we use is a sampling protocol due to Braverman [5], in fact a modification of it used by Kerenidis, Laplante, Lerays, Roland and Xiao [23].

2 Introduction Given a model of computation, suppose solving one instance of a given problem f with probability of success p < requires c units of some resource. A natural question that may be asked is: how much resource is needed to solve f k, k instances of the same problem, simultaneously. A naive way is by running the optimal protocol for f, k times in parallel, which requires c k units of resource, however the probability of overall success is p k exponentially small in k). A strong direct product conjecture for f states that this is essentially optimal, that is if only ok c) units of resource are provided for any protocol solving f k, then the probability of overall success is at most p Ωk). Proving or disproving strong direct product conjectures in various models of computation has been a central task in theoretical computer science, notable examples of such results being Yao s XOR lemma [38] and Raz s [29] theorem for two-prover games. Readers may refer to [25, 4, 8] for a good discussion of known results in different models of computation. In the present work, we consider the model of two-party two-way public-coin communication complexity [37] and consider the direct product question in this model. In this model, there are two parties who wish to compute a joint function more generally a relation) of their input, by doing local computation, sharing public coins and exchanging messages. The resource counted is the number of bits communicated between them. The textbook by Kushilevitz and Nisan [26] is an excellent reference for communication complexity. Much effort has been made towards investigating direct product questions in this model and strong direct product theorems have been shown for many different functions, for example Set-Disjointness [25, 4], Inner Product [27], Pointer Chasing [8] etc. To the best of our knowledge, it is not known if the strong direct product conjecture fails to hold for any function or relation in this model. Therefore, whether the strong direct product conjecture holds for all relations in this model, remains one of the major open problems in communication complexity. In the model of constant-round public-coin communication complexity, recently a strong direct product result has been shown to hold for all relations by Jain, Perezlényi and Yao [8]. The work [8] built on a previous result due to Jain [4] showing a strong direct product result for all relations in the model of one-way public-coin communication complexity where a single message is sent from Alice to Bob, who then determines the answer). The weaker direct sum conjecture, which states that solving k independent instances of a problem with constant success probability requires k times the resource needed to compute one instance with constant success probability, has also been extensively investigated in different models of communication complexity and has met a better success. Direct sum theorems have been shown to hold for all relations in the public-coin one-way model [2], the entanglementassisted quantum one-way model [20], the public-coin simultaneous message passing model [2], the private-coin simultaneous message passing model [5], the constant-round public-coin twoway model [6] and the model of two-way distributional communication complexity under product distributions [3]. Again please refer to [25, 4, 8] for a good discussion. Another major focus in communication complexity has been to investigate generic lower bound methods, that apply to all functions and possibly to all relations). In the model we are concerned with, various generic lower bound methods are known, for example the partition bound [6], the information complexity [9], the smooth rectangle bound [6] which in turn subsumes the rectangle bound a.k.a the corruption bound) [36,, 3, 24, 4], the smooth discrepancy bound a.k.a the γ 2 bound [28] which in turn subsumes the discrepancy bound), the subdistribution bound [7] and the conditional min-entropy bound [4]. Proving strong direct product results in terms of these lower bound methods is a reasonable approach to attacking the general question. Indeed, many lower bounds have been shown to satisfy strong direct product theorems, example the discrepancy bound [27], the subdistribution bound under product distributions [7], the smooth discrepancy bound [34] and the conditional min-entropy bound [4].

3 Our result In present work, we show a strong direct product theorem in terms of the smooth rectangle bound, introduced by Jain and Klauck [6], which generalizes the rectangle bound a.k.a. the corruption bound) [36,, 3, 24, 4]. Roughly speaking, the rectangle bound for relation f X Y Z under a distribution µ, with respect to an element z Z, and error ε, tries to capture the size under µ) of a largest rectangle for which z is a right answer for ε fraction of inputs inside the rectangle. It is not hard to argue that the rectangle bound forms a lower bound on the distributional communication complexity of f under µ. The smooth rectangle bound for f further captures the maximum, over all relations g that are close to f under µ, of the rectangle bound of g under µ. The distributional error setting can eventually be related to the worst case error setting via the well known Yao s principle [36]. Jain and Klauck showed that the smooth rectangle bound is stronger than every lower bound method we mentioned above except the partition bound and the information complexity. Jain and Klauck showed that the partition bound subsumes the smooth rectangle bound and in a recent work Kerenidis, Laplante, Lerays, Roland and Xiao [23] showed that the information complexity subsumes the smooth rectangle bound building on the work of Braverman and Weinstein [7] who showed that the information complexity subsumes the discrepancy bound). New lower bounds for specific functions have been discovered using the smooth rectangle bound, for example Chakrabarti and Regev s [8] optimal lower bound for the Gap-Hamming Distance partial function. Klauck [25] used the smooth rectangle bound to show a strong direct product result for the Set-Disjointness function, via exhibiting a lower bound on a related function. On the other hand, as far as we know, no function or relation) is known for which its smooth rectangle bound is asymptotically) strictly smaller than its two-way public-coin communication complexity. Hence establishing whether or not the smooth rectangle bound is a tight lower bound for all functions and relations in this model is an important open question. Our result is as follows. Theorem.. 2 Let X, Y, Z be finite sets, f X Y Z be a relation. Let µ be a distribution on X Y. Let z Z and β = Pr x,y) µ [fx, y) = {z}]. Let ε, δ > 0. There exists a small enough ε > 0 such that the following holds. For all integers t, R pub ε) ε2 t/32 f t ) ε2 32 t ε srec z,µ +ε )δ/β,δ f) 2 ). Above R pub ) represents the two-way public-coin communication communication complexity and srec ) represents the smooth rectangle bound please refer to Section 2 for precise initions). Our result implies a strong direct product theorem for all relations for which an asymptotically) optimal lower bound can be provided using the smooth rectangle bound. Jain and Klauck [6] provided an alternate inition Definition.3) of the smooth rectangle bound for partial) functions in terms of linear programs and show a relationship between the two initions. We show a tighter relationship between the two initions in Lemma.4. Combining Lemma.4 with Theorem. we get the following result. Theorem.2. Let f : X Y Z be a partial) function. For every ɛ 0, ), there exists small enough η 0, /3) such that the following holds. For all integers t, R pub f t) ) η η2 η) η2 t/32 32 t log srec ɛ f) 3 log ɛ ) 2. For a relation f with R pub ε f) = O), a strong direct product result can be shown via direct arguments [4]. 2 With a slight abuse of notation, we write fx, y) = {z Z x, y, z) f}, and f z) = {x, y) : x, y, z) f}. 2

4 As a consequence, our results reprove some of the known strong direct product results, for example for Inner Product [26] and Set-Disjointness [25, 4]. Recently smooth rectangle bound has been used to provide new tight lower bounds for several functions, for example for the Gap- Hamming Distance [8, 33] partial function and the Greater-Than function [35]. These results, along with our result, imply strong direct product for these functions. Smooth rectangle bound has also been used to provide near optimal lower bounds for several important functions and relations used to show exponential separations between classical and quantum communication complexity for example by Raz [30], Gavinsky [] and Klartag and Regev [32]. These results combined with our result imply near optimal strong direct product results for these functions and relations. In a recent work, Harsha and Jain [2] have shown that the smooth-rectangle bound provides an optimal lower bound of Ωn) for the Tribes function. For this function all other weaker lower bound methods mentioned before like the rectangle bound, the sub-distribution bound, the smooth discrepancy bound, the conditional min-entropy bound etc. fail to provide an optimal lower bound since they are all O n). Earlier Jayram, Kumar and Sivakumar [22] had shown a lower bound of Ωn) using information complexity. The result of [2] along with Theorem.2 implies a strong direct product result for the Tribes function. This adds to the growing list of functions for which a strong direct product result can be shown via Theorem.2. In [23], Kerenidis et. al. introduced the relaxed partition bound a weaker version of the partition bound [6]) and showed it to be stronger than the smooth rectangle bound. It is easily seen by comparing the corresponding linear-programs) that the smooth rectangle bound and the relaxed partition bound are in-fact equivalent for boolean functions and more generally when the size of output set is a constant). Thus our result also implies a strong direct product theorem in terms of the relaxed partition bound for boolean functions and more generally when the size of output set is a constant). Our techniques The broad argument of the proof of our result is as follows. We show our result in the distributional error setting and translate it to the worst case error setting using the well known Yao s principle [36]. Let f be a relation, µ be a distribution on X Y, and c be the smooth rectangle bound of f under the distribution µ with output z Z. Consider a protocol Π which computes f k with inputs drawn from distribution µ k and communication oc k) bits. Let C be a subset of the coordinates {, 2,..., k}. If the probability that Π computes all the instances in C correctly is as small as desired, then we are done. Otherwise, we exhibit a new coordinate j / C, such that the probability, conditioned on success in C, of the protocol Π answering correctly in the j-th coordinate is bounded away from. Since µ could be a non-product distribution we introduce a new random variable R j, such that conditioned on it and X j Y j input in the jth coordinate), Alice and Bob s inputs in the other coordinates become independent. Use of such a variable to handle non product distributions has been used in many previous works, for example [2, 3, 3, 4, 8]. Let the random variables Xj Y j R j M represent the inputs in the jth coordinate, the new variable R j and the message transcript of Π, conditioned on the success on C. The first useful property that we observe is that the joint distribution of Xj Y j R j M can be written as, Pr [ X j Y j R j M = xym ] = q µx, y)u xr j, m)u y r j, m), where u x, u y are functions and q is a positive real number. The marginal distribution of Xj Y j is no longer µ though. However using arguments as in [4, 8]), one can show that the distribution of Xj Y j is close, in l distance, to µ and I Xj ; R j M ) Y j + I Y j ; Rj M ) X j oc), where I; ) represents the mutual information please refer to Section 2 for precise initions). 3

5 Now, assume for contradiction that the success in the jth coordinate in Π is large, like 0.99, conditioned on success in C. Using the conditions obtained in the previous paragraph, we argue that there exists a zero-communication public-coin protocol Π, between Alice and Bob, with inputs drawn from µ. In Π Alice and Bob are allowed to abort the protocol or output an element in Z. We show that the probability of non-abort for this protocol is large, like 2 c, and conditioned on non-abort, the probability that Alice and Bob output a correct answer for their inputs is also large, like This allows us to exhibit by fixing the public coins of Π appropriately), a large rectangle with weight under µ like 2 c ) such that z is a correct answer for a large fraction like 0.99) of the inputs inside the rectangle. This shows that the rectangle bound of f, under µ with output z, is smaller than c. With careful analysis we are also able to show that the smooth rectangle bound of f under µ, with output z, is smaller than c, reaching a contradiction to the inition of c. The sampling protocol that we use to obtain the public-coin zero communication protocol, is the same as that in Kerenidis et al. [23], which in turn is a modification of a protocol due to Braverman [5] 3 a variation of which also appears in [7]). However our analysis of the protocol s correctness deviates significantly in parts from the earlier works [23, 5, 7] due to the fact that for us the marginal distribution of X Y need not be the same as that of µ, in fact for some inputs x, y), the probability under the two distributions can be significantly different. There is another important original contribution of our work, not present in the previous works [23, 5, 7]. We observe a crucial property of the protocol Π which turns out to be very important in our arguments. The property is that the bad inputs x, y) for which the distribution of Π s sample for Rj M, conditioned on non-abort, deviates a lot from the desired Rj M X Y = xy), their probability is nicely reduced as compared to Pr [ X Y = xy ] ) in the final distribution of Π, conditioned on non-abort. This helps us to argue that the distribution of inputs and outputs in Π, conditioned on non-abort, is close in l distance to X j Y j R j M, implying good success in Π, conditioned on non-abort. Organization. In Section 2, we present some necessary background, initions and preliminaries. In Section 3, we prove our main result Theorem.. We er some proofs to Appendix. 2 Preliminary Information theory We use capital letters e.g. X, Y, Z or letters in bold e.g. a, b, α, β to represent random variables and use calligraphic letters e.g. X, Y, Z to represent sets. For integer n, let [n] represent the set {, 2,..., n}. Let X, Y be finite sets and k be a natural number. Let X k be the set X X, the cross product of X, k times. Let µ be a probability) distribution on X. Let µx) represent the probability of x X according to µ. For any subset S X, ine µs) = x S µx). Let X be a random variable distributed according to µ, which we denote by X µ. We use the same symbol to represent a random variable and its distribution whenever it is clear from the context. The expectation value of function f on X is denoted as E x X [fx)] = x X Pr[X = x] fx). The entropy of X is ined as HX) = x µx) log µx) log, ln represent logarithm to the base 2, e repectively). For two distributions µ, λ on X, the distribution µ λ is ined as µ λ)x, x 2 ) = µx ) λx 2 ). Let µ k = µ µ, k times. The l distance between µ and λ is ined to be half of the l norm of µ λ; that is, λ µ = 2 x λx) µx) = max S X λs) µs). We say that λ is ε-close to µ if λ µ ε. The relative entropy 3 A protocol, achieving similar task, however working only for product distributions on inputs was first shown by Jain, Radhakrishnan and Sen [20]. 4

6 [ between distributions X and Y on X is ined as SX Y ) = E x X { min-entropy between them is ined as S X Y ) = max x X log Pr[X=x] Pr[Y =x] log Pr[X=x] Pr[Y =x] ]. The relative }. It is easy to see that SX Y ) S X Y ). Let X, Y, Z be jointly distributed random variables. Let Y x denote the distribution of Y conditioned on X = x. The conditional entropy of Y conditioned on X is ined as HY X) = E x X [HY x )] = HXY ) HX). The mutual information between X and Y is ined as: IX ; Y ) = HX) + HY ) HXY ) = E y Y [SX y X)] = E x X [SY x Y )]. The conditional mutual information between X and Y, conditioned on Z, is ined as: IX ; Y Z) = E z Z [IX ; Y Z = z)] = H X Z) + H Y Z) H XY Z). The following chain rule for mutual information is easily seen : IX ; Y Z) = IX ; Z) + IX ; Y Z). We will need the following basic facts. A very good text for reference on information theory is [0]. Fact 2.. Relative entropy is jointly convex in its arguments. That is, for distributions µ, µ, λ, λ X and p [0, ]: S pµ + p)µ ) λ + p)λ p Sµ λ)+ p) S µ ) λ. Fact 2.2. Relative entropy satisfies the following chain rule. Let XY and X Y be random variables on X Y. It holds that: S X Y XY ) = S X X ) +Ex X [ S Y x Yx )]. In particular, S X Y X Y ) = S X X ) +E x X [ S Y x Y )] S X X ) +S Y Y ). The last inequality follows from Fact 2.. Fact 2.3. Let XY and X Y be random variables on X Y. It holds that S X Y X Y ) S X Y X Y ) = I X ; Y ). The following fact follows from Fact 2.2 and Fact 2.3. Fact 2.4. Given random variables XY and X Y on X Y, it holds that Fact 2.5. For distributions λ and µ: E x X [SY x Y )] E x X [SY x Y )] = IX ; Y ). 0 λ µ Sλ µ). Fact 2.6. Classical substate theorem [9]) Let X, X be two distributions on X. For any δ 0, ), it holds that [ Pr[X ] = x] Pr x X Pr[X = x] 2SX X)+)/δ δ. We will need the following lemma. Its proof is erred to Appendix. Lemma 2.7. Given random variables A, A and ε > 0, if A A ε, then for any r 0, ), [ ] Pr a A Pr[A =a] Pr[A=a] ε r 2r; and [ ] Pr a A Pr[A =a] Pr[A=a] ε r 2r ε. Communication complexity Let X, Y, Z be finite sets, f X Y Z be a relation and ε > 0. In a two-way public-coin communication protocol, Alice is given x X, and Bob is given y Y. They are supposed to output z Z such that x, y, z) f via exchanging messages and doing local computations. They may share public coins before the inputs are revealed to them. We assume that the last 5

7 log Z bits of the transcript is the output of the protocol. Let R pub ε f) represent the twoway public-coin randomized communication complexity of f with the worst case error ε, that is the communication of the best two-way public-coin protocol for f with error for each input x, y) being at most ε. Let µ be a distribution on X Y. Let D µ ε f) represent the two-way distributional communication complexity of f under distribution µ with distributional error ε, that is the communication of the best two-way deterministic protocol for f, with average error over the distribution of the inputs drawn from µ, at most ε. Following is Yao s min-max principle which connects the worst case error and the distributional error settings, see. e.g., [26, Theorem 3.20, page 36]. Fact 2.8. [37] R pub ε f) = max µ D µ ε f). The following fact can be easily verified by induction on the number of message exchanges in a private-coin protocol please refer for example to [5] for an explicit proof). It is also implicit in the cut and paste property of private-coins protocol used in Bar-Yossef, Jayram, Kumar and Sivakumar [2]. Lemma 2.9. For any private-coin two-way communication protocol, with input XY µ and transcript M M, the joint distribution can be written as Pr[XY M = xym] = µx, y)u x m)u y m), where u x : M [0, ] and u y : M [0, ], for all x, y) X Y. Smooth rectangle bound Let f X Y Z be a relation and ε, δ 0. With a slight abuse of notation, we write fx, y) = {z Z x, y, z) f}, and f z) = {x, y) : x, y, z) f}. Definition 2.0. Smooth-rectangle bound [6]) The ε, δ)-smooth rectangle bound of f, denoted by srec ε,δ f), is ined as follows: srec ε,δ f) = max{ srec λ ε,δ f) λ a distribution over X Y}; srec λ ε,δ f) = max{ srec z,λ f) z Z}; ε,δ srec z,λ ε,δ f) = max{ rec z,λ ε g) g X Y Z; Pr x,y) λ [fx, y) gx, y)] δ}; rec z,λ ε g) = min{s λ R λ) R is a rectangle in X Y, λg z) R) ε)λr)}. When δ = 0, the smooth rectangle bound equals the rectangle bound a.k.a. the corruption bound) [36,, 3, 24, 4]. Definition 2.0 is a generalization of the one in [6], where it is only ined for boolean functions. The smooth rectangle bound is a lower bound on the two-way public-coin communication complexity. The proof of the following lemma appears in Appendix. Lemma 2.. Let f X Y Z be a relation. Let λ X Y be a distribution and let z Z. Let β = Pr x,y) λ [fx, y) = {z}]. Let ε, ε δ+ε, δ > 0 be such that β 2ε < + ε ) δ β. Then, R ε f) D λ ε f) srec z,λ +ε )δ/β,δ f) log 4 ε. 6

8 3 Proof The following lemma builds a connection between the zero-communication protocols and the smooth rectangle bound. Lemma 3.. Let f X Y Z, X Y X Y be a distribution and z Z. Let β = Pr x,y) X Y [fx, y) = {z}]. Let c. Let ε, ε, δ > 0 be such that δ+2ε)/β 3ε) < +ε )δ/β. Let Π be a zero-communication public-coin protocol with input X Y, public coin R, Alice s output A Z { }, and Bob s output B Z { }. Let X Y A B R = X Y ABR A = B ). Let. Pr[A = B ] 2 c ; 2. X Y X Y ε. 3. Pr [ X, Y, A ) f ] ε. Then srec z,x Y +ε )δ/β,δ f) < c ε. Proof. Let g X Y Z, satisfy Pr x,y) X Y [fx, y) gx, y)] δ. It suffices to show that rec z,x Y +ε )δ/β g) c ε. Since Pr[A = B ] 2 c, c S X Y R A B X Y RAB ) S X Y R A B X Y RAB ) Since X Y X Y ε, E r R,a A [ S X Y ) r,a X Y )] from Fact 2.2). ) Pr xyr X Y R[fx, y) = {z}] Pr xy X Y [fx, y) = {z}] ε β ε. 2) Since Pr [ X, Y, A ) f ] ε, hence Pr [ A = B = z ] β 2ε. Since by item 2 of this lemma, we have Pr x,y) X Y [fx, y) gx, y)] δ, Pr xyra X Y R A[x, y, a) g] Pr xyra X Y R A[x, y, a) f] δ ε 2ε δ. 3) By standard application of Markov s inequality on equations ), 2), 3), we get an r 0, such that S X Y ) r0,z X Y ) c ε, Pr [z / gx, y)] δ + 2ε)/β 3ε) + ε )δ/β. xy X Y ) r0,z Here, X Y ) r0,z = X Y R = r 0, A = z). Note that the distribution of X Y ) r0,z is the distribution of X Y restricted to some rectangle and then rescaled to make a distribution. Hence S X Y ) r0,z X Y ) = S X Y ) r0,z X Y ). Thus rec z,x Y +ε )δ/β g) < c ε. The following is our main lemma. A key tool that we use here is a sampling protocol that appears in [23] protocol Π as shown in Figure ), which is a variant of a sampling protocol that appears in [7], which in turn is a variant of a sampling protocol that appears in [5]. Naturally 7

9 similar arguments and calculations, as in this lemma, are made in previous works [5, 7, 23], however with a key difference. In their setting m u xm)u y m) = for all x, y). However in our setting this number could be much smaller than one for different x, y). Hence our arguments and calculations deviate from previous works at several places significantly. Another important original contribution of our work is Claim 3.7 which is used in the proof of the main lemma. We highlight its importance later just before its proof. Lemma 3.2. Main Lemma) Let c. Let p be a distribution over X Y and z Z. Let = Pr x,y) p [fx, y) = {z}]. Let 0 < ε < /3 and δ, ε > 0 be such that δ+22ε β 33ε < + ε ) δ β. Let XY M be random variables jointly distributed over the set X Y M such that the last log Z bits of M represents an element in Z. Let u x : M [0, ], u y : M [0, ] be functions for all x, y) X Y. If it holds that, β. For all x, y, m) X Y M, Pr [XY M = xym] = q px, y)u xm)u y m), where q = xym px, y)u xm)u y m); 2. SXY p) ε 2 /4; 3. IX ; M Y ) + IY ; M X) c; 4. err f XY M) ε, where err f XY M) = Pr xym XY M [x, y, m) / f], and m represents the last log Z bits of m; then srec z,p +ε )δ/β,δ f) < 2c ε 3. Proof. Note by direct calculations, Pr[XY = xy] = q px, y)α xy, where α xy = m u x m)u y m); ; 4) Pr[X = x] = q px)α x, where α x = y py x)α xy ; 5) Pr[Y = y] = q py)α y, where α y = x px y)α xy ; 6) Pr[X y = x] = px y)α xy, Pr[Y x = y] = py x)α xy ; α x α y 7) Pr[M xy = m] = u x m)u y m)/α xy ; 8) Pr[M x = m] = u xm)v x m) α x, where v x m) = y py x)u ym); 9) Pr[M y = m] = u ym)v y m) α y, where v y m) = x px y)u xm). 0) Define G = {x, y) : G 2 α xy q 2 and α x q 2 and α xy q }; ) 2 = {x, y) : SM xy M x ) + SM xy M y ) c/ε} ; 2) [ G uy m) = {x, y) : Pr m M xy v x m) 2 and u ] xm) v y m) 2 2ε}. 3) We begin by showing that G G 2 is a large set and also G G 2 G. Claim Pr x,y) p [x, y) G ] > 6ε, 2. Pr x,y) p [x, y) G 2 ] 3ε/2, 8

10 Alice s input is x. Bob s input is y. Common input is c, ε, q, M.. Alice and Bob both set = c/ε+ ε + 2, T = 2 q M 2 ln ε 2. For i =,, T : and k = log 3 ε ln ε )). a) Alice and Bob, using public coins, jointly sample m i M, α i, β i [0, 2 ], uniformly. b) Alice accepts m i if α i u x m i ), and β i 2 v x m i ). c) Bob accepts m i if α i 2 v y m i ), and β i u y m i ). 3. Let A = {i [T ] : Alice accepts m i } and B = {i [T ] : Bob accepts m i }. 4. Alice and Bob, using public coins, choose a uniformly random function h : M {0, } k and a uniformly random string r {0, } k. a) Alice outputs if either A is empty or hm i ) r where i is the smallest element in non-empty A). Otherwise, she outputs the element in Z, represented by the last log Z bits of m i. b) Bob finds the smallest j B such that hm j ) = r. If no such j exists, he outputs. Otherwise, he outputs the element in Z, represented by the last log Z bits of m j. Figure : Protocol Π 3. Pr x,y) p [x, y) G G 2 ] 5ε/2, 4. G G 2 G. Proof. Note item. and item 2. imply item 3. Now we show. Note that using item 2. of Lemma 3.2 and Fact 2.5) XY p ε/2. From Lemma 2.7 and 4), we have [ Pr α ] xy x,y) p q /2 2ε. By the monotonicity of l -norm, we have X p X ε 2 and X p Y ε 2. Similarly, from 5) and 6) we have Pr x,y) p [ α x q ] /2 2ε, and Pr By the union bound, item. follows. Next we show 2. From item 3. of Lemma 3.2, x,y) p [ α y q ] /2 2ε. E [SM xy M x ) + SM xy M y )] = IX ; M Y ) + IY ; M X) c. x,y) XY Markov s inequality implies Pr x,y) XY [x, y) G 2 ] ε. Then item 2. follows from the fact that XY and p are ε/2-close. 9

11 Finally we show 4. For any x, y) G G 2, SM xy M x ) c/ε [ ] Pr[Mxy = m] Pr 2 c/ε+ ε ε from Fact 2.6) m M xy Pr[M x = m] [ ] uy m)α x Pr 2 c/ε+ ε ε from 8) and 9)) m M xy v x m)α xy [ ] uy m) Pr m M xy v x m) 2 ε. x, y) G and the choice of ) Similarly, Pr m Mxy [ uxm) v ym) 2 ] ε. By the union bound, [ uy m) Pr m M xy v x m) 2 and u ] xm) v y m) 2 2ε, which implies x, y) G. Hence G G 2 G. Following few claims establish the desired properties of protocol Π Figure ). Definition 3.4. Define the following events. ˆ E occurs if the smallest i A satisfies hm i ) = r and i B. Note that E implies A. ˆ B c subevent of E) occurs if E occurs and there exist j B such that hm j ) = r and m i m j, where i is the smallest element in A. ˆ H = E B c. Below we use conditioning on x, y) as shorthand for Alice s input is x and Bob s input is y. Claim 3.5. For any x, y) G G 2, we have. for all i [T ], and 2 2 q M 2 Pr q M 2 Pr [Alice accepts m i x, y)] 3 2 [Bob accepts m i x, y)] 3 2 where is the internal randomness of protocol Π ; 2. Pr rπ [B c x, y), E] ε; 3. Pr rπ [H x, y)] 4ε) 2 k 2. q M 2, q M 2, Proof.. We do the argument for Alice. Similar argument follows for Bob. Note that u x m), v x m) [0, ]. Then for all x, y) X Y, Pr[Alice accepts m i x, y)] = u x m)v x m) M 2 = α x M 2. m Item follows by the fact that x, y) G. 0

12 2. Define E i subevent of E) when i is the smallest element of A. For all x, y) G G 2, we have : Pr[B c x, y), E i ] = Pr[ j : j B and hm j ) = r and m j m i x, y), E i ] Pr[j B and hm j ) = r and m j m i x, y), E i ] from the union bound) j [T ],j i Pr j [T ],j i T ε. [j B x, y), E i ] Pr[hm j ) = r x, y), E i, j B, m j m i ] 3q M k two-wise independence of h and item. of this Claim) from choice of parameters) Since above holds for every i, it implies Pr rπ [B c x, y), E] ε. 3. Consider, Pr[E x, y)] = Pr 2 [A x, y)] Pr[E A, x, y)] ) ) T q M 2 Pr[E A, x, y)] ε) Pr[E A, x, y)] from choice of parameters) using item. of this Claim) = ε) Pr[hm i ) = r A, x, y)] Pr[i B i A, hm i ) = r, x, y)] from here on we condition on i being the first element of A) = ε) 2 k Pr[i B i A, x, y)] = ε) 2 k Pr [i B and i A x, y)] Pr rπ [i A x, y)] 2 3q ε) 2 k M 2 Pr[i B and i A x, y)] using item. of this claim) = 2 3q ε) 2 k M 2 M 2 2 min { u x m), 2 v y m) } min { u y m), 2 v x m) } m M from construction of protocol Π ) 2 3q ε) 2 k M 2 u x m)u y m) M 2 2 m G xy = 2 3q ε) 2 k M 2 α xy M 2 2 G xy u x m)u y m) α xy m G xy 3 ε) 2 k Pr [m G xy ] since x, y) G and 8)) m M xy = {m : u x m) 2 v y m) and u y m) 2 v x m)}) 3 ε) 2 k 2ε) since x, y) G, using item 4. of Claim 3.3) 3ε) 2 k 2.

13 Finally, using item 2. of this Claim. Pr [H x, y)] = Pr [E x, y)] Pr[B c x, y), E]) 4ε) 2 k 2. Claim 3.6. Pr p,rπ [H] 23 2 ε) 2 k 2. Proof. Pr [H] p, x,y) G G 2 px, y) Pr 23 2 ε) 2 k 2. [H x, y)] 4ε) 2 k 2 x,y) G G 2 px, y) The second inequality is by Claim 3.5, item 3, and the last inequality is by Claim 3.3 item 3. The following claim is an important original contribution of this work not present in the previous works [23, 5, 7].) The claim helps us establish a crucial property of Π. The property is that the bad inputs x, y) for which the distribution of Π s sample for M, conditioned on nonabort, deviates a lot from the desired, their probability is nicely reduced in the final distribution of Π, conditioned on non-abort. This helps us to argue that the joint distribution of inputs and the transcript in Π, conditioned on non-abort, is still close in l distance to XY M. Claim 3.7. Let AB and A B be random variables over A B and h : A [0, + ) be a function. Suppose for any a A, there exist functions f a, g a : B [0, + ), such that. a,b ha)f ab) =, and Pr[AB = ab] = ha)f a b); 2. f a b) g a b), for all a, b) A B ; 3. Pr[A B = ab] = ha)g a b)/c, where C = a,b ha)g ab); 4. Pr a A [Pr b Ba [f a b) = g a b)] δ ] δ 2, for δ [0, ), δ 2 [0, ). Then AB A B δ + δ 2. Proof. Set G = {a, b) : f a b) = g a b)}. By condition 4, Pr a,b) AB [a, b) G] δ δ 2. Then C = a,b ha)g a b) a,b:a,b) G ha)f a b) = Pr [a, b) G] δ δ 2. 4) a,b) AB We have AB A B = ha)f a b) 2 C ha)g ab) a,b ha)f a b) ha)g a b) + ha)g a b) ) 2 C ha)g ab) a,b ha)f a b) ha)g a b)) + C ha)g a b) using item 2. of this claim) 2 C a,b a,b ha)f a b) + C 2 a,b:a,b) / G = ) Pr [a, b) / G] + C δ + δ 2 from 4)) 2 a,b) AB 2

14 Claim 3.8. Let the input of protocol Π be drawn according to p. Let X Y M represent the input and the transcript the part of the public coins drawn from M) conditioned on H. Then we have XY M X Y M 0ε. Note that this implies that X Y A B XY M M 0ε, where M represents the last log Z bits of M and A, B represent outputs of Alice and Bob respectively, conditioned on H. Proof. For any x, y), ine w xy m) = min { u x m), 2 v y m) } min { u y m), 2 v x m) }. From step 2 a),b),c), of protocol Π, Pr [ M X Y = mxy ] = C px, y)w xym), where C = xym px, y)w xym). Now, Pr x,y) XY [ Prm Mxy [w xy m) = u x m)u y m)] 2ε ] = Pr x,y) XY [x, y) G] 8ε. The last inequality above follows using items 3. and 4. of Claim 3.3 and the fact that XY and p are ε/2-close. Finally using Claim 3.7 by substituting δ 2ε, δ 2 8ε, A XY, B M, A X Y, B M, h p q, f x,y)m) u x m)u y m) and g x,y) m) w xy m)), we get that X Y M XY M 0ε. We are now ready to finish the proof of Lemma 3.2. Consider the protocol Π. We claim that it satisfies Lemma 3. by taking the correspondence between quantities in Lemma 3. and Lemma 3.2 as follows : c c/ε 2 + 3/ε), ε ε, β β, δ δ, z z, X Y p. Item. of Lemma 3. is implied by Claim 3.6 since 23 2 ε) 2 k 2 2 c/ε2 +3/ε), from choice of parameters. Item 2. of Lemma 3. is implied since X Y p X Y XY + XY p 2 2 ε, using item 2. of Lemma 3.2, Fact 2.5 and Claim 3.8. Item 3. of Lemma 3. is implied since err f X Y M ) err f XY M)+ X Y M XY M ε, using item 4. in Lemma 3.2 and Claim 3.8. This implies srec z,p +ε )δ/β,δ f) < c/ε2 + 3/ε 2c ε ε 3. We can now prove our main result. Theorem 3.9. Let X, Y, Z be finite sets, f X Y Z be a relation. Let µ be a distribution on X Y. Let z Z and β = Pr x,y) µ [fx, y) = {z}]. Let 0 < ε < /3 and ε, δ > 0 be such that δ+22ε β 33ε < + ε ) δ β. For all integers t, R pub ε) ε2 t/32 f t ) ε2 32 t ε srec z,µ +ε )δ/β,δ f) 2 ). Proof. Set δ = ε 2 /32. ine c = ε srec z,µ +ε )δ/β,δ f) 2 and XY µt. By Fact 2.8, it suffices to show D µt ε) ε2 t/32 f t ) δ tc. Let Π be a deterministic two-way communication protocol, that computes f t, with total communication δ ct bits. The following claim implies that the success of Π is at most ε) δt, and this shows the desired. 3

15 Claim 3.0. For each i [t], ine a binary random variable T i {0, }, which represents the success of Π on the i-th instance. That is, T i = if the protocol computes the i-th instance of f correctly, and T i = 0 otherwise. Let t = δ t. There exists t coordinates {i,, i t } such that for each r t,. either Pr [ T r) = ] ε) t or 2. Pr [ T ir+ = T r) = ] r) ε, where T = r j= T i j. Proof. Suppose we have already identified r coordinates, i,, i r satisfying that Pr[T i ] ε and Pr [ T ij+ = T j) = ] ε for j r. If Pr [ T r) = ] ε) t, then we are done. So from now on we assume Pr [ T r) = ] > ε) t 2 δt. Here we assume r. Similar arguments also work when r = 0, that is for identifying the first coordinate, which we skip for the sake of avoiding repetition. Let D be a random variable uniformly distributed in {0, } t and independent of XY. Let U i = X i if D i = 0, and U i = Y i if D i =. For any random variable L, ine L = L T r) = ). If L = L L t, ine L i = L L i L i+ L t. Let C = {i,, i r }. Define R i = D i U i X C [i ] Y C [i ]. Now let us apply Lemma 3.2 by substituting XY Xj Y j, M R j M, p X j Y j, z z, ε ε, δ δ, β β, ε ε and c 6δ c + ). Condition. in Lemma 3.2 is implied by Claim 3.. Conditions 2. and 3. are implied by Claim 3.2. Also we have srec z,µ +ε )δ/β,δ f) > 32δ c+) ε, by our choice of c. Hence condition 4. must be false and hence err 3 f X j Yj M ) = err f X j Yj R j M ) > ε. This shows condition 2. of this Claim. Claim 3.. Let R denote the space of R j. There exist functions u xj, u yj : R M [0, ] for all x j, y j ) X Y and a real number q > 0 such that Pr [ X j Y j R j M = x j y j r j m ] = q µx j, y j )u xj r j, m)u yj r j, m). Proof. Note that X j Y j is independent of R j. Now consider a private-coin two-way protocol Π with input X j Y j as follows. Let Alice generate R j and send to Bob. Alice and Bob then generate X j ) xjr j and Y j ) yjr j, respectively. Then they run the protocol Π. Thus, from Lemma 2.9, Pr[X j Y j R j M = xy j rm] = µx j, y j ) v xj r j, m) v yj r j, m), where v xj, v yj : R M [0, ], for all x j, y j ) X Y. Note that conditioning on T r) = corresponds to choosing a subset, say S, of R M. Let q = x jy jr jm:r j,m) S µx j, y j )v xj r j, m)v yj r j, m). Then Pr [ X j Y j R j M = x j y j r j m ] = q µx j, y j )v xj r j, m)v yj r j, m), for r j, m) S and Pr [ Xj Y j R j M = x j y j r j m ] = 0 otherwise. Now ine u xj r j, m) = v xj r j, m), and u yj r j, m) = v yj r j, m), for r j, m) S and ine them to be 0 otherwise. The claim follows. 4

16 Claim 3.2. If Pr [ T r) = ] > 2 δt, then there exists a coordinate j / C such that and S Xj Y ) Xj Y j 8δ = ε2 4, 5) j I Xj ; M ) ) Rj Y j + I Y j ; M Rj X j 6δ c + ). Proof. This follows using Claim III.6 in [8]. We include a proof in Appendix for completeness. 6) Conclusion and open problems We provide a strong direct product result for the two-way public-coin communication complexity in terms of an important and widely used lower bound method, the smooth rectangle bound. Some natural questions that arise are:. Is the smooth rectangle bound a tight lower bound for the two-way public-coin communication complexity for all relations? If yes, this would imply a strong direct product result for the two-way public-coin communication complexity for all relations, settling a major open question in this area. To start with we can ask: Is the smooth rectangle bound a polynomially tight lower bound for the two-way public-coin communication complexity for all relations? 2. Or on the other hand, can we exhibit a relation for which the smooth rectangle bound is asymptotically) strictly smaller than its two-way public-coin communication complexity? 3. Can we show similar direct product results in terms of possibly stronger lower bound methods like the partition bound and the information complexity? 4. It will be interesting to obtain new optimal lower bounds for interesting functions and relations using the smooth rectangle bound, implying strong direct product results for them. Acknowledgement. We thank Prahladh Harsha for helpful discussions. References [] Laszlo Babai, Peter Frankl, and Janos Simon. Complexity classes in communication complexity theory. In Proceedings of the 27th Annual Symposium on Foundations of Computer Science, SFCS 86, pages , Washington, DC, USA, 986. IEEE Computer Society. [2] Ziv Bar-Yossef, T. S. Jayram, Ravi Kumar, and D. Sivakumar. An information statistics approach to data stream and communication complexity. In Proceedings of the 43rd Symposium on Foundations of Computer Science, FOCS 02, pages , Washington, DC, USA, IEEE Computer Society. [3] Boaz Barak, Mark Braverman, Xi Chen, and Anup Rao. How to compress interactive communication. In Proceedings of the 42nd ACM symposium on Theory of computing, STOC 0, pages 67 76, New York, NY, USA, 200. ACM. [4] Paul Beame, Toniann Pitassi, Nathan Segerlind, and Avi Wigderson. A strong direct product theorem for corruption and the multiparty communication complexity of disjointness. Comput. Complex., 54):39 432, December [5] Mark Braverman. Interactive information complexity. In Proceedings of the 44th annual ACM symposium on Theory of computing, STOC 2, pages , New York, NY, USA, 202. ACM. 5

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19 [36] Andrew C. Yao. Lower bounds by probabilistic arguments. In Proceedings of the 24th Annual Symposium on Foundations of Computer Science, SFCS 83, pages , Washington, DC, USA, 983. IEEE Computer Society. [37] Andrew Chi-Chih Yao. Some complexity questions related to distributive computing preliminary report). In Proceedings of the eleventh annual ACM symposium on Theory of computing, STOC 79, pages , New York, NY, USA, 979. ACM. [38] Andrew Chi-Chih Yao. Theory and applications of trapdoor functions. In Proceedings of the 23RD Symposium on Foundations of Computer Science, FOCS 82, pages 80 9, Washington, DC, USA, 982. IEEE Computer Society. Proof of Lemma 2.7: Let G = { a : Pr[A =a] Pr[A=a] ε r }, then 2ε Pr[A = a] Pr[A = a] Pr[A = a] Pr[A = a] a a G = Pr[A = a] Pr[A = a] Pr[A = a] Pr ε [a G] a A r. a G Thus Pr a A [a G] 2r. The second inequality follows immediately. Proof of Lemma 2.: Let c = srec z,λ f). Let g be such that +ε ) δ recz,λ g) = c and β,δ +ε ) δ β Pr x,y) λ [fx, y) gx, y)] δ. If D λ ε f) c log4/ε) then we are done using Fact 2.8. So lets assume for contradiction that D λ ε f) < c log4/ε). This implies that there exists a deterministic protocol Π for f with communication c log4/ε) and distributional error under λ bounded by ε. Since Pr x,y) λ [fx, y) gx, y)] δ, the protocol Π will have distributional error at most ε + δ for g. Let M represent the message transcript of Π and let O represent protocol s output. We assume that the last log Z bits of M contain O. We have,. Pr m M [Pr[M = m] 2 c ] ε/4, since the total number of message transcripts in Π is at most 2 c log4/ε). 2. Pr m M [O = z M = m] > β ε, since Pr x,y) λ [fx, y) = {z}] = β and distributional error of Π under λ is bounded by ε for f. ] 3. Pr m M [Pr x,y) XY )m [x, y, O) / g M = m] ε+δ β 2ε β 2ε, since distributional error of Π under λ is bounded by ε + δ for g. Using all of above we obtain a message transcript m such that Pr [M = m] > 2 c and O = z M = m) and ε + δ Pr [x, y, O) / g M = m] x,y) XY M=m) β 2ε < + ε ) δ β. This and the fact that the support of XY M = m) is a rectangle, implies that rec z,λ g) < c, +ε ) δ β contradicting the inition of c. Hence it must be that D λ ε f) c log4/ε), which using Fact 2.8 shows the desired. Proof of Claim 3.2: The following calculations are helpful for achieving 5). δ k > S X Y XY ) S X Y XY ) i / C S Xi Y ) X i Y i, 7) i 8

20 where the first inequality follows from the assumption that Pr [ T r) = ] > 2 δk, and the last inequality follows from Fact 2.2. The following calculations are helpful for 6). δ k > S X Y D U XY DU ) S X Y D U ) XY DU E d,u,x C,y C ) D,U,X C,Y C = i/ C = i/ C = 2 [ S X Y ) )] XY ) d,u,xc,yc d,u,xc,y C E d,u,x C [i ],y C [i ] ) D,U,X C [i ],Y C [i ] E d i,u i,r i) D i,u i,r i i/ C E r i,x i) R i,x i [ X S i Yi ) d,u,x C [i ], y C [i ] [ S X i Y i ) di,u i,r i Xi Y i ) di,u i,r i )] [ Y ) )] Yi S i r i,x i ) xi + 2 i / C 8) ) ] X iy i ) d,u,xc [i ], 9) y C [i ] 20) [ X )] Xi E S i ) )ri,yi r i,y i) R i,y yi. i 2) Above, Eq. 8) and Eq. 9) follow from Fact 2.2; Eq. 20) is from the inition of R i. Eq. 2) follows since Di is independent of R i and with probability half D i is 0, in which case U i = X i and with probability half Di is in which case U i = Y i. By Fact 2.4, I X i ; Ri ) Y i + I Y i ; Ri )) X i 2δ k. 22) i/ C We also need the following calculations, which exhibits that the information carried by messages about sender s input is small. δ ck M I X Y ; M D U XCY C ) ) I Xi Yi ; M D U XC [i ] Y C [i ] = i/ C = i/ C = 2 i/ C I Xi Yi ; M D i Ui Ri ) I X i ; M R i Yi ) + I Y i ; M R i Xi )). 23) Above first equality follows from chain rule for mutual information, second equality follows from inition of Ri and the third equality follows since with probability half D i is 0, in which case Ui = X i and with probability half Di is in which case U i = Y i. Combining Eqs. 7), 2) and 23), and making standard use of Markov s inequality, we can get a coordinate j / C such that S Xj Yj ) X j Y j 8δ, I Xj ; Rj ) Y j + I Y j ; Rj ) X j 6δ, I Xj ; M R j Yj ) + I Y j ; M R j Xj ) 6δ c. The first inequality is exactly the same as Eq. 5). Eq. 6) follows by adding the last two inequalities. 9