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{{Orphan|date=December 2024}}
'''Permutation codes''' are a family of [[Error correction model|error correction
== Definition and properties ==
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Let <math>D(n,k)= \{ \sigma \in S_n: d_H (\sigma, id)=k\}</math> with <math>|D(n,k)|=\tbinom{n}{k}D_k</math>, where <math>D_k</math> is the number of [[derangement]]s of order <math>k</math>.
The [[Gilbert–Varshamov bound|Gilbert-Varshamov bound]] is a very well known upper bound,<ref name=":0">{{Cite journal |last1=Gao |first1=Fei |last2=Yang |first2=Yiting |last3=Ge |first3=Gennian |date=May 2013 |title=An Improvement on the Gilbert–Varshamov Bound for Permutation Codes |url=http://dx.doi.org/10.1109/tit.2013.2237945 |journal=IEEE Transactions on Information Theory |volume=59 |issue=5 |pages=3059–3063 |doi=10.1109/tit.2013.2237945 |bibcode=2013ITIT...59.3059G |s2cid=13397633 |issn=0018-9448|url-access=subscription }}</ref> and so far outperforms other bounds for small values of <math>d</math>.
'''Theorem 1''': <math>\frac{n!}{\sum _{k=0} ^{d-1} |D(n,k)|} \leq M(n,d) \leq \frac{n!}{\sum _{k=0} ^{[\frac{d-1}{2}]} |D(n,k)|}
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<math>\frac{n!}{M(n,4)} \geq 1 + \frac{(n+1)n(n-1)}{n(n-1)-(n-k^2)((k+1)^2-n)((k+2)(k-1)-n)}</math>.
For small values of <math>n</math> and <math>d</math>, researchers have developed various computer searching strategies to directly look for permutation codes with some prescribed [[automorphism]]s <ref>{{Cite journal |last1=Smith |first1=Derek H. |last2=Montemanni |first2=Roberto |date=2011-08-19 |title=A new table of permutation codes |url=http://dx.doi.org/10.1007/s10623-011-9551-8 |journal=Designs, Codes and Cryptography |volume=63 |issue=2 |pages=241–253 |doi=10.1007/s10623-011-9551-8 |s2cid=207115236 |issn=0925-1022|hdl=11380/1176210 |hdl-access=free }}</ref>
== Other Bounds ==
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