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Suppose we have an $n$ by $n$ chessboard and each square can be in two states: "on" or "off". At time $t=0$ all bits are off. Every second one square is selected at random uniformly out of the $n^2$ squares and the corresponding bit is flipped. How to define entropy of "on" bits in this process and show that it increases? This came out of a model of a gas in a container, where we divide the container using a grid. Each grid point is "on" or "off" iff there is a gas molecule in the corresponding square. It should increase on average right? Seems to me it could be related to the variance of the "on" squares but I am no expert in the field, any hints?

Thanks in advance!

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    $\begingroup$ This is relevant. In that form, it is only applicable to a $1$-dimensional random variable. I'm not sure how to generalize to multiple dimensions. $\endgroup$
    – Guy
    May 31, 2017 at 18:53
  • $\begingroup$ Why a chessboard? Why not an $n\times n$ grid? Am I missing something? $\endgroup$
    – Shaun
    May 31, 2017 at 18:53
  • $\begingroup$ I don't think so. I've seen chessboard being used to mean grid before, and while it does seem odd, I've gotten used to it. $\endgroup$
    – Guy
    May 31, 2017 at 18:54
  • $\begingroup$ I think one issue here is that you want some measure of how mixed the bits are, as well as how many are on and off, which is perhaps a different question than just entropy. To that end, this may help. $\endgroup$ May 31, 2017 at 18:59

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A simple measure is just the number of configurations of the on bits. If $n$ bits are on there are $64 \choose n$ configurations they can be in, so you can say the entropy is $\log {64 \choose n}$. Initially $n$ increases rapidly because the chance you turn off an on bit is small. As $n$ approaches $32$ you will approach maximum entropy. There doesn't seem to be anything in the problem that involves the particular pattern of bits, so I wouldn't do anything more complicated.

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  • $\begingroup$ Thanks! simpler than I thought.. $\endgroup$
    – plus1
    Jun 16, 2017 at 16:59

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