How many independent yes/no questions can be asked about a point in binary space (linear vs nonlinear codes)?

cs.stackexchange https://cs.stackexchange.com/questions/53813

  •  03-11-2019
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Pergunta

This question springs from thinking about the potential benefits of using nonlinear codes instead of linear codes. Say we have a point $x \in \{0,1\}^k$ and we want to guess what it is. A naive scheme would be to ask $2^k$ questions of the form "is $x=[0100001\dots 01]$?" etc, while a refinement would be "is the $i$:th bit a one?" while linear codes generalizes this one step further to "is $x_i + x_j + \dots + x_\ell = 1$?"

With linear codes we can enumerate $k$ independent questions of this form. What I'm wondering is: can we do better?

Call $\{0,1\}^k$ our codebook with a corresponding matrix $C$ containing the coordinates of the codewords in its rows, i.e. $C_0 = [0\dots 0], C_1 = [0\dots 0 1]$ etc. Say we permute this matrix and make the permutation known to the receiver prior to transmission.

Couldn't we produce more than $k$ independent questions by asking questions of the form "is $b_i + b_j + \dots = 1$?", where $b_i$ is the $i$:th bit of the binary representation of the permuted codebook index?

It seems this could generalize into any "type division": to form a question, divide the codebook into $A_i$ and $B_i$ where $|A_i| = |B_i|$ and ask questions of the form "is $x$ of type $A_i$ or $B_i$?"

Clearly, this scheme is a strict generalization of linear coding since parity checks follow this general even division algorithm. Is it a vacuous generalization or could we in fact produce more than $k$ independent questions in this manner?

EDIT: Clarifying independence due to demand - could we by using nonlinear coding construct a list of $k+2$ or more questions such that any size $k$ subset would recover the unique answer? (I made it $k+2$ since I think we can construct a list of $k+1$ questions by using linear codes - just make the $k+1$:th question about the parity of all the rest)

EDIT: This is not a question on decoding tractability, so please refrain on commenting on that aspect.

Nenhuma solução correta

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