# Finding a majority ball with majority answers

### Mate Vizer MTA Alfred Renyi Institute of Mathematics

#### Gabor Wiener Department of Computer Science and Information Theory, Budapest University of Technology and Economics

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Minisymposium: GENERAL SESSION TALKS

Content: Suppose we are given a set of $n$ balls $\{b_1,\ldots,b_n\}$ each colored either red or blue in some way unknown to us. To find out some information about the colors, we can query any triple of balls $\{b_{i_1},b_{i_2},b_{i_3}\}$. As an answer to such a query we obtain (the index of) a {\em majority ball}, that is, a ball whose color is the same as the color of another ball from the triple. Our goal is to find a {\em non-minority ball}, that is, a ball whose color occurs at least $\frac n2$ times among the $n$ balls. We show that the minimum number of queries needed to solve this problem is $\Theta(n)$ in the adaptive case and $\Theta(n^3)$ inthe non-adaptive case. We also consider some related problems.

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