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Compactly representing integers when allowed a multiplicative error

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Problem Detail: 

Consider the problem of representing in memory numbers in the range $\{1,\ldots,n\}$.

Obviously, exact representation of such number requires $\lceil\log_2(n)\rceil$ bits.

In contrast, assume we are allowed to have compressed representations such that when reading the number we get a 2-approximation for the original number. Now we can encode each number using $O(\log\log n)$ bits. For example, given an integer $x\in\{1,\ldots,n\}$, we can store $z = \text{Round}(\log_2 x)$. When asked to reconstruct an approximation for $x$, we compute $\widetilde{x} = 2^z$. Obviously, $z$ is in the range $\{0,1,\ldots,\log_2 n\}$ and only requires $O(\log\log n)$ bits to represent.

In general, given a parameter $\epsilon>0$, what is the minimal number of bits required for saving an approximate representation of an integer in the above set, so that we can reconstruct its value up to a multiplicative error of $(1+\epsilon)$?

The above example shows that for $\epsilon=1$, approximately $\log\log n$ bits are enough. This probably holds asymptotically for every constant $\epsilon$. But how does epsilon affect the memory (e.g., does it require $\Theta(\frac{1}{\epsilon^2}\log\log n)$ bits?).

Asked By : R B
Answered By : D.W.

Storing $x$ to within a $1+\epsilon$ approximation can be done with $\lg \lg n - \lg(\epsilon) + O(1)$ bits.

Given an integer $x$, you can store $z = \text{Round}(\log_{1+\epsilon} x)$. $z$ is in the range $\{0,1,\dots,\log_{1+\epsilon} n\}$, so requires about $b = \lg \log_{1+\epsilon} n$ bits. Doing a bit of math, we find

$$b = \lg \frac{\lg n}{\lg(1+\epsilon)} = \lg \lg n - \lg \lg (1+\epsilon).$$

Now $\lg(1+\epsilon) = \log(1+\epsilon)/\log(2) \approx \epsilon/\log(2)$, by a Taylor series approximation. Plugging in, we get

$$b = \lg \lg n - \lg(\epsilon) + \lg(\log(2)),$$

which yields the claimed answer.

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Question Source : http://cs.stackexchange.com/questions/53633

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