A special invertible k-bits to n-bits mapping

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Discussion Overview

The discussion centers on the problem of finding an invertible mapping from k-bits to n-bits that produces a sequence of n bits without the all 1's output. Participants explore both linear and non-linear mappings, seeking to determine the minimum number of bits k required for various values of n, particularly focusing on the constraints and characteristics of these mappings.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes a need for a mapping that excludes the all 1's sequence from the output, questioning the minimum k required for such a mapping given n.
  • Another participant points out the lack of clarity regarding the domain of the mapping and the definition of "using" bits in the context of the mapping.
  • A participant defines the domain and range of the mapping, suggesting that for linear mappings, k is constrained to n-1, and questions the existence of non-linear mappings that could meet the criteria.
  • One participant asserts that no invertible mappings can exist that meet the criteria, emphasizing that an invertible mapping must be one-to-one, thus the range cannot have a smaller cardinality than the domain.

Areas of Agreement / Disagreement

Participants express differing views on the existence of non-linear invertible mappings that can produce the desired output. While some agree on the constraints of linear mappings, there is contention regarding the possibility of non-linear alternatives.

Contextual Notes

Participants note that the definitions of mapping and transform may not be strictly adhered to, which could affect the clarity of the discussion. Additionally, the implications of cardinality in relation to invertibility are highlighted as a critical aspect of the problem.

ait.abd
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I want a sequence of [itex]n[/itex] bits without all [itex]1[/itex]'s in the output. What is the minimum number of bits [itex]k[/itex] given [itex]n[/itex] that can be used in any linear or non-linear invertible mapping that will produce such a sequence at output. For example, consider [itex]n=3[/itex]. I want to create a mapping that has all [itex]2^3=8[/itex] minus [itex]111[/itex] sequence. What is the minimum number of bits [itex]k[/itex] required that will produce such a mapping? In this example, for a linear mapping, I can see that [itex]k=2[/itex]. But with [itex]k=2[/itex], I am loosing [itex]3[/itex] sequences at the output and the overhead of this mapping [itex]k/n[/itex] is huge. Is there any other method or non-linear mapping that does this job with an overhead of [itex]k/n>2/3[/itex]. I need a specific answer to [itex]n=4[/itex] or general answer with any [itex]n[/itex] (reasonable range for me is [itex]n=2[/itex] to [itex]n=16[/itex]. Please guide me to any keywords that may help in my search for the answer.
 
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ait.abd said:
I want a sequence of [itex]n[/itex] bits without all [itex]1[/itex]'s in the output. What is the minimum number of bits [itex]k[/itex] given [itex]n[/itex] that can be used in any linear or non-linear invertible mapping that will produce such a sequence at output.

You haven't defined the domain of mapping.
You haven't explained what it means to "use" bits in the mapping.
 
Domain of mapping is [itex]S=\{\text{All } 2^k \text{ sequences of k-bits}\}[/itex].
Range of the mapping is [itex]T=\{2^n-1 \text{ sequences of n-bits i.e. All sequences of n-bits excluding the all 1's sequence}\}[/itex]
By using I mean defining a mapping S to T. In case of linear mapping, the answer is straightforward that the maximum number of bits [itex]k[/itex] required are [itex]n-1[/itex]. My question is whether there are any non-linear invertible mappings/transforms that take [itex]k[/itex] bits and output a sequence of length [itex]k[/itex] bits (not [itex]n[/itex]) that doesn't contain all 1's sequence. I don't see any linear mapping fulfilling this criterion as [itex]k \le (n-1)[/itex] is always true for linear mappings/transform.
*I may not be using mapping and transform words in the strict sense. By mapping or transform I mean is there any [itex]t = f(s); t \in T, s \in S; t=t_1,t_2,t_3,...,t_k; s=s_1,s_2,s_3,...,s_n; s_p,t_p \in \{0,1\}[/itex]
 
ait.abd said:
My question is whether there are any non-linear invertible mappings/transforms that take [itex]k[/itex] bits and output a sequence of length [itex]k[/itex] bits (not [itex]n[/itex]) that doesn't contain all 1's sequence.

There are no invertible mappings like that. An invertible mapping has to be 1-to-1. The range can't have a smaller cardinality than the domain.
 

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