Abstract
In \({[k]^n=[k]{\times}[k]{\times}\cdots{\times}[k]}\), a coordinate line consists of the collection of points where all but one coordinate is fixed and the unfixed coordinate varies over all possibilities. We consider the problem of marking (or designating) one point on each line in [k]n so that each point in [k]n is marked either a or b times, for some fixed a or b. This is equivalent to forming a strategy for a hat guessing game for n players with k different colors of hats where the number of correct guesses, regardless of hats placed, is either a or b. If we let s ≥ 0 and t ≥ 0 denote the number of vertices marked a and b times respectively, then we have the following obvious necessary conditions: s + t = k n (the number of points) and as + bt = nk n–1 (the number of lines). Our main result is to show for n ≤ 5, and k arbitrary, that these necessary conditions are also sufficient.
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This is one of several papers published together in Designs, Codes and Cryptography on the special topic: “Combinatorics—A Special Issue Dedicated to the 65th Birthday of Richard Wilson.”
This work was done with support of an NSF Mathematical Sciences Postdoctoral Fellowship.
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Butler, S., Graham, R. A note on marking lines in [k]n . Des. Codes Cryptogr. 65, 165–175 (2012). https://doi.org/10.1007/s10623-011-9507-z
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DOI: https://doi.org/10.1007/s10623-011-9507-z