Talk:Gödel numbering for sequences

message: \overline n denotes \{0, \dots, n-1\} here? It's not so widely accepted notation, thus we way want to provide a definition for it. しんのすけ代表 (talk) 03:56, 5 March 2020 (UTC)
thread_title:  Need definition of <math>\overline n</math> 

message: What does "more ergonomic approach" (paragraph 3 in the current version) mean? -- Walt Pohl (talk) 14:05, 18 November 2009 (UTC)
thread_title:  Ergonomic? 

message: I think the (very long) section about Gödel's β function does not belong in this article. For one thing, Gödel's β function does not encode a finite sequence of integers into one integer but rather decodes two integers (and an implicit length indication) into a finite sequence of values. But more to the point, the focus of Gödel's β function is not to encode sequences into numbers for the purpose of algorithmic manipulation at all. When that is necessary, Gödel encodes sequences as exponents in a prime factorization instead, which (although computationally expensive) allows straightforward algorithmic encoding and decoding.
However Gödel (still) needs his β function separately as a tool when considering arithmetic definability. When considering arithmetically definable sets, one has at the basis polynomial expressions as a means to define sets, not algorithms. Then in order to show that nevertheless (graphs of) primitive recursive functions are arithmetically definable, one needs a means to express the fact that a given graph satisfies a certain primitive recursive relation, which naturally involves a relation of any point of the graph with the full set of previous points in the graph. Finitely quantified arithmetic formulas cannot directly refer to that whole set of previous values, but thanks to the β function, quantification over just two integers will suffice to get the same result as quantification over such sequences. All that is necessary is that the β function is sufficiently flexible to,  by appropriately fixing the first two arguments, produce arbitrary finite sequences of values when varying its third argument. However it must also be obviously arithmetically definable (without having to use the fact that all primitive recursive functions are), and this is why encoding by prime factorization won't do in this context. Once it is proved that the β function has this flexibility, there is no need to actually compute those two values that encode for a given finite sequence, since there will be quantification over all pairs of natural numbers for its first two arguments.
So unless somebody argues strongly to the contrary, I will remove the unhelpful parts of this article (maybe move some useful parts not already there to the beta function page), and replace them by the discussion of a simple encoding by pairing scheme, which for algorithmic purposes suffices, while being much more transparent. Marc van Leeuwen (talk) 15:54, 6 February 2012 (UTC)
replies: text: I think it makes the most sense for this article to focus on several different ways of encoding sequences. The Β function is one (since there are polynomial bijections that can reduce two numbers to a single number), the prime power coding is another, and there are probably others in computer science. — Carl (CBM · talk) 16:23, 6 February 2012 (UTC)
thread_title:  Section about Gödel's ''β'' function is not appropriate here 

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message: I don't get why the part on coprimality of the moduli m_i=im+1,1\leqslant i\leqslant n has to be so unnecessarily formal. The whole "Proof that (coprimality) assumption for Chinese remainder theorem is met" section can be made a one-liner. Why not just say : 
If m is divisible by 1,\dots,n then the m_i are pairwise coprime.
Proof. Suppose a prime p divides m_i,m_j for some 1\leqslant i<j\leqslant n. Then p divides m_j-m_i=(j-i)m, so p divides j-i or m. Since it divides m_i=im+1, it cannot divide m, so it must divide j-i. But j-i is an integer between 1 and n, so p divides m, contradiction. 37.166.24.201 (talk) 08:59, 29 November 2023 (UTC)
thread_title:  Why so verbose ?