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A180632
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Minimum length of a string over the alphabet A = {1,2,...,n} that contains every permutation of A as a substring exactly once, also known as length of the minimal super-permutation.
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11
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OFFSET
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0,3
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COMMENTS
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Obviously bounded below by n! + n - 1 and above by 2(n! - (n - 1)!) + 1.
A better lower bound is n! + (n - 1)! + (n - 2)! + n - 3 and a better upper bound is A007489(n). - Nathaniel Johnston, Apr 22 2013
The above mentioned lower bound was essentially shown in 2011 by an anonymous poster on the internet and, filling in some minor details, brought into a formal form by Houston, Pantone and Vatter (see reference). - Peter Luschny, Oct 27 2018
Different from the minimal supersequence, in which each permutations of n letters can appear as a subsequence instead of a sub-string (i.e., with noncontiguous characters). Refer to A062714. - Maurizio De Leo, Mar 02 2015
In October 2018 Greg Egan found new records for n=7, 8, 9: a(7) <= 5908, a(8) <= 46205, and a(9) <= 408966. More generally, for any n >= 7, a(n) <= n! + (n-1)! + (n-2)! + (n-3)! + n - 3. - Peter Luschny, Oct 26 2018; corrected by Max Alekseyev, Jan 07 2019
In February 2019, Bogdan Coanda found an example showing that a(7) <= 5907. Later the same month, Greg Egan found an example showing a(7) <= 5906. - Robin Houston, Mar 11 2019
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REFERENCES
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D. Ashlock and J. Tillotson. Construction of small superpermutations and minimal injective superstrings. Congressus Numerantium, 93 (1993), 91-98.
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LINKS
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Miryana Stefanović, Supermutacije (Supermutations), Master's Thesis, Univ. Belgrade (Serbia 2023). See p. 9. (In Serbian)
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EXAMPLE
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For n = 1, 2, 3, 4, the (unique, up to relabeling the symbols) minimal words are:
1
121
123121321
123412314231243121342132413214321
For n = 5, there are exactly 8 distinct (up to relabeling the symbols) minimal words.
Comment from N. J. A. Sloane, Mar 27 2015: From the Houston (2014 arXiv) paper, a superpermutation of length 872 (not known to be minimal, but shorter than the old upper bound of 873):
1234561234516234512634512364513264513624513642513645213645123465123415 6234152634152364152346152341652341256341253641253461253416253412653412 3564123546123541623541263541236541326543126453162435162431562431652431 6254316245316425314625314265314256314253614253164523146523145623145263 1452361452316453216453126435126431526431256432156423154623154263154236 1542316542315642135642153624153621453621543621534621354621345621346521 3462513462153642156342165342163542163452163425163421564325164325614325 6413256431265432165432615342613542613452613425613426513426153246513246 5312463512463152463125463215463251463254163254613254631245632145632415 6324516324561324563124653214653241653246153264153261453261543265143625 1436521435621435261435216435214635214365124361524361254361245361243561 2436514235614235164235146235142635142365143265413625413652413562413526 41352461352416352413654213654123
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CROSSREFS
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KEYWORD
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nonn,hard,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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