# Questions tagged [information-theory]

Questions about Information theory, entropy, and information content of various sources

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### Binary compression algorithm with decompress by index

I have a list of 256-bit binary data to store. Any algorithm for doing lossless compression on it in a way I can retrieve an entry by its index without decompressing the whole data (if possible). The ...
1 vote
27 views

### Chain rule for mutual information

I am a bit confused in the following definition: what does comma mean. I know I(X;Y) is the mutual information between X and Y but I am not sure how to interpret the I(X_1,X_2;Y)).
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1 vote
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### Undecidability in optimal data compression

There is this certain slide in Coursera Computer Science: Algorithms, Theory, and Machines course: I think it is saying finding the optimal size of given data is undecidable. However, I thought there ...
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1 vote
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### Is there a method for information transfer without copying the data?

What I mean is, if in real life I had an object and I gave it to you, I would not have it anymore. I did not give you a copy of the object. Is there a way to do that with information?
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### How to compress highly correlated arrays?

I want to write a compression algorithm for a specific use case that I have. I have many arrays which are for the most part, very similar. Each value in the array is an integer and is related to the ...
1 vote
1k views

### What is the name of the following binary encoding?

S is a the set of binary strings in Shortlex order: [0,1,00,01,10,11,...] I want to encode / decode natural numbers with the following scheme: Encoding N: For ...
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1 vote
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### What does it mean for an algorithm to not resample points?

The No Free Lunch theorems for search and optimization demonstrate that for search/optimization problems in a limited search space, where the points being searched through are not resampled, the ...
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### If all words have the same frequency, is the generated Huffman tree a balanced binary tree?

If all words have the same frequency, is the generated Huffman tree a balanced binary tree? At the same time, is the generated Huffman tree a complete binary tree?
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1 vote
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### Noisy channel coding: how to design a code for a discrete channel

When determining the capacity of a discrete channel, we maximise mutual information, and in doing so find the optimal input distribution for that channel. I know that the noisy-channel coding theorem ...
1 vote
44 views

### Entropy of $\{X_1,\ldots,X_T\}$ where $X_1,\ldots,X_T \in_R [Q]$

I have a noiseless T-user multiple access channel. Let $[Q] = \{1, \dots , Q \}$. The users send symbols $X_i ∈[Q], i = 1, \dots , T$ . The channel is defined as follows $$(X_1, X_2, \dots , X_T) → Y$$...
59 views

### Upper bound of sum of codewords lengths

I need to show that for any binary optimal code for $n$-letter source the following inequality holds: $$\sum_{i=1}^n l_i \leq 0.5(n + 2)(n −1).$$ By $l_i$ denoted the length of the sequence ...
1 vote
43 views

### Sorting lower bound is linear when only a constant number of distinct keys?

I saw this proposition on Sedgewick's lecture slides on QuickSort, and I've been wondering why the number of comparisons is linear in the case of a constant number of distinct keys. I tried to ...
11 views

### What does it exactly mean by information management as a function of O/s

I have read that information management is a function of O/s by which it A) Manages, stores, retrieves,modify data. It makes sure that right people have access to their information. B) Some of the ...
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### How many code alphabets do we need in order for a Huffman Code and a Shannon-Fano Code to be the same for the same source symbols probability

For example, If we have a source with alphabet $\{x, y, z\}$ with probabilities $\{0.5, 0.3, 0.2\}$ , What is the smallest integer $\mathbf{D}$ such that the expected length of a D-ary Shannon-Fano ...
1 vote
34 views

### How to decode shortened Reed-Solomon code?

I am working with a shortened version of $[n,k,d]$ Reed-Solomon code. I am encoding a message of size $k−l$ which gives a shortened code of size $n−l$ (this is equivalent to encoding the same message ...
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### Is there an entropy evaluation method with an unified length?

For entropy, the most common one is Shannon entropy, however, it ignores time series of data. For instance, data 0x00001111 and 0x01010101 are given the same entropy. It is obvious that the second ...
15 views

### What is the difference between erasure coding and RAID if your erasure code is parity check?

I'm reading about erasure coding and saw that one of the erasure codes is parity check. As far as I can tell this is just a generalization of something like RAID5. If you select something like parity ...
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### Proof in the "Reaching Agreement in the Presence of Faults"

I am reading the "Reaching Agreement in the Presence of Faults", M. Pease et al and trying to understand their proof for the $n \geq 3m+1$ case. In the induction step $m \gt 0$ it says the ...
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### Information theory - Expurgation step to go from average error to worst case error in the large error regime

Consider a discrete memoryless channel $N$. We use a code to send messages over this channel. Shannon showed that if we have a code $C$ with a finite number of codewords $|C|$ such that the average ...
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1 vote
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### What is the entropy of an unordered list?

I'm trying to compress unordered lists of a few thousand integers for transmission over HTTP, and Claude Shannon is disappointing me with his mathematical ambiguity :) Each integer is 6-digits, so ...
1 vote
66 views

### Is every binary sequence the output of some meaningful-text-input algorithm?

Here is the problem: Let's say we have a random binary sequence, just an arbitrary sequence of zeroes and ones (of some arbitrary length of digits). Can we find an algorithm that would decode/...
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1 vote
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### Is an AI upscaler incapable of reducing entropy?

I was reading the description of Anime4K (a video upscaler software) and I found a statement triggering my attention: [upscaling is done] without any meaningful decrease in entropy (lost information ...
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### Deriving a lower bound on the conditional entropy, conditioned on an event

I came across Lemma 19 in Certifying Equality With Limited Interaction, which states the following for jointly distributed random variables $Z$, $W$, where $Z$ takes values in $\{0,1\}^n$, and some ...
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### Asymptotically Optimal Universal Code In Other Bases

Universal codes are fairly well studied, and many asymptotically optimal universal codes exist for binary data (see https://en.wikipedia.org/wiki/Universal_code_(data_compression) especially https://...
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### Capacity of Binary Erasure Channel

Consider the the binary erasure channel, with input and output alphabet $\{0,?,1\}$ and channel matrix \begin{bmatrix} 1-\lambda-\mu & \mu & \lambda\\ 0 & 1 & 0\\ \lambda & \mu &...
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1 vote
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### Why does attempting to estimate the entropy of text by randomly choosing chars in it and counting how often they are equal give wildly wrong results?

Why does attempting to estimate the entropy of a string, by randomly choosing pairs of (not necessarily adjacent) characters in it, and counting how often the selected characters in the pairs are ...
31 views

### Essential bit content - prove that we can't use less bits than that

Let $H_0$ be $\log_2(|A|)$, where $A$ is a set. Let $C$ be a compressor $C\colon A \to \{1,0\}^l \cup \bot$. This is a silly question, because intuitively it seems obvious. How can I prove that $l$ ...
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1 vote
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### Algorithmic information theory with stochastic algorithms?

Suppose we define a class of algorithms that is allowed to sample i.i.d. Bernoulli bitstrings of arbitrary length, and use these to generate outputs. If we are allowed to use algorithms like this, ...
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### How Data Compression relates to Estimating Distribution?

I recently read this paper Mahoney, 1999. And encountered this line, optimal compression of a probabilistic language L with unknown distribution (such as English) using an estimated distribution M (...
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### How to calculate the entropy of a system with multiple states

I'm stuck in trying to compute an overall entropy calculation with an agent. Let me first introduce some background of the problem. Basically, I'm doing some work with the contextual bandit problems. ...
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### Information-theoretic lower bound for succinct string dictionary of the Unicode Name property

Background The literature on succinct data structures refers often to the “information-theoretic lower bound” of encoding data, i.e., the minimum number of bits needed to store the data – a concept ...
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### Maximal prefix codes and maximal length

Let $X$ a maximal prefix code on an alphabet $A$, $m(X)$ its maximal length, $F = X \cap A^{m(X)}$ and $F’ \subseteq A^{m(X)}$. Let $X’ = X \setminus F \cup F’$ a maximal prefix code. Why is it true ...
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### what is the relationship between entropy and variance?

Consider a simple Bernoulli variable X X = 1 with probability p X = 0 with probability (1-p) The variance is simply p(1-p). The ...
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1 vote
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### Must a Turing machine tape be binary?

I once asked why does computer data bits are usually organized on binary (base 2) sets, rather than on unary (base 1) sets, aiming to also understand why its not also ternary (base 3), heptary (base 7)...
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### Shared randomness does not increase capacity of a noisy channel - Why?

Why is it the case that when Alice and Bob use a noisy channel for communication, the capacity of the channel does not increase even if they are allowed to share pre-distributed randomness? This is ...
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1 vote
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### How to interpret parametric formulation of information bottleneck?

I'm reading this paper on latent representations with the information bottleneck https://arxiv.org/pdf/1804.06216.pdf and in section three, the authors write that the parametric formulation of the ...
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### How to build 4 codewords with a code distance of 5?

I wonder how can I construct 4 (distinct) codewords given the fact that code distance is 5. As far as I know that the code distance is the number of distinct bits between any 2 codewords. How to ...