Technical Report CS0870

Authors: E. Kushilevitz, N. Linial and R. Ostrovsky
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Abstract: Consider a network of $k+1$ processors connected by $k$ links in a {\em linear array}. The processors wish to compute some boolean function $f(x,y)$ where each of $x$ and $y$ is stored in one of the end-processors of the array. Let $D_k(f)$ be the (total) number of bits that must be exchanged to compute $f$. Clearly, $D_k(f) \leq k \cdot D(f)$, where $D(f)$ is the standard two party communication complexity of $f$. Tiwari proved that for almost all functions $D_k(f) \geq k (D(f) - O(1))$ and conjectured that this is true for all functions. In this paper we disprove Tiwari's conjecture by exhibiting a function for which $D_k(f)$ is essentially at most $\frac{3}{4}k \cdot D(f)$. Using our result we also get some progress on another important problem in communication complexity: How good are lower bounds proved by giving lower bounds on the number of monochromatic rectangles required to partition the space of inputs. We show that for certain functions the (two-party) communication complexity may be 2 times larger than the best lower bound obtained this way.
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