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Search: id:A102381
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| A102381 |
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Number of permutations of 1..n in which every pair of adjacent numbers as well as the first and the last entries are relatively prime. |
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+0
4
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| 1, 2, 6, 8, 60, 24, 504, 576, 6480, 5760, 242352, 93312, 6200064, 5612544, 95294880, 136249344, 13687492608, 5022425088, 693149184000, 472559616000, 18501259714560, 23441203298304, 4435759798272000, 1568692666368000
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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a(n)=n*A086595(n).
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LINKS
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Leroy Quet, Home Page (listed in lieu of email address)
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EXAMPLE
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a(4)=8 because we have 1234, 1432, 2143, 2341, 3214, 3412, 4123 and 4321.
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MAPLE
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with(combinat): for n from 1 to 7 do P:=permute(n): ct:=0: for j from 1 to n! do if add(gcd(P[j][i+1], P[j][i]), i=1..n-1)=n-1 and gcd(P[j][1], P[j][n])=1 then ct:=ct+1 else ct:=ct fi od: a[n]:=ct: od: seq(a[n], n=1..7);
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CROSSREFS
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Cf. A086595, A076220.
Sequence in context: A098239 A140257 A053938 this_sequence A075998 A007849 A100621
Adjacent sequences: A102378 A102379 A102380 this_sequence A102382 A102383 A102384
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KEYWORD
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nonn
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AUTHOR
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Emeric Deutsch (in collaboration with R. Chandler, V. Jovovic, L. Quet, Z. Seidov and J. Zucker) (deutsch(AT)duke.poly.edu), Apr 09 2005
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EXTENSIONS
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a(15)=95294880 and a(16)=136249344 from Ray Chandler (rayjchandler(AT)sbcglobal.net) and Joshua Zucker (joshua.zucker(AT)stanfordalumni.org), Apr 12 2005
Many more terms from Max Alekseyev (maxale(AT)gmail.com), Jun 13 2005
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