The function $\mathrm{INDEX}:\{0,1\}^n\times\{1,\dots,n\}\to \{0,1\}$ is defined as

$$\mathrm{INDEX}(x,i)=x_i,$$ where $x=x_1\dots x_n$. I am looking for the randomized communication complexity of $\mathrm{INDEX}$ for an arbitrary number of rounds.

Somehow, I do not manage to find it in the literature.

The deterministic communication complexity is known to be $\Theta(\log n)$, while its one-way randomized communication complexity is $\Theta(n)$.

I guess the randomized communication complexity is still $\Theta(\log n)$, I was not able to find a lower bound.

Since this function its well-studied, it should be stated somewhere.

Thanks for your help.

  • $\begingroup$ There should be a simple information-theoretic argument. Discrepancy might also work. $\endgroup$ – Yuval Filmus Jan 24 '19 at 13:17
  • $\begingroup$ The result itself should follow from known lifting theorems, but that’s overkill. $\endgroup$ – Yuval Filmus Jan 24 '19 at 13:18
  • $\begingroup$ Average or worst case complexity? $\endgroup$ – orlp Jan 24 '19 at 15:41
  • $\begingroup$ Communication complexity is always worst case. $\endgroup$ – Yuval Filmus Jan 24 '19 at 17:38

It is known that the randomized communication complexity of inner product on $m$ bits is $\Omega(m)$. You can compute inner product using a protocol for the indexing function on $\{0,1\}^{2^m} \times [2^m]$ as follows: denoting Alice's input by $x \in \{0,1\}^m$ and Bob's by $y \in \{0,1\}^m$, Alice computes a new vector $X$ by $X_y = \mathsf{IP}(x,y)$, and the parties run the indexing protocol on the inputs $X$ and $y$. This gives an $\Omega(m) = \Omega(\log n)$ lower bound for indexing.


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