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Search: id:A091217
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| A091217 |
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Number of pairs of permutations of [n] such that the sums of all adjacent elements are unique. |
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+0
3
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| 1, 2, 6, 16, 54, 232, 1132, 6024, 36262, 242080, 1775316
(list; graph; listen)
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OFFSET
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1,2
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EXAMPLE
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For a(4), we have 2+3=1+4, so a(4)=4! - (23)(14) - (14)(32) = 24 - 4 - 4 = 16.
For a(5), we have 1+4=2+3, 1+5=2+4, 2+5=3+4 to avoid.
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PROGRAM
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(PARI) { for (i=3, 9, v=vector(i-1); c=0; for (j=1, i!, x=numtoperm(i, j); for (k=1, i-1, v[k]=x[k]+x[k+1]); fl=0; v=vecsort(v); for (z=1, i-2, if (v[z]==v[z+1], fl=1; break)); if (fl==0, c++)); print1(", "c)) }
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CROSSREFS
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Adjacent sequences: A091214 A091215 A091216 this_sequence A091218 A091219 A091220
Sequence in context: A100664 A003446 A045696 this_sequence A068787 A073959 A006820
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KEYWORD
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more,nonn
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AUTHOR
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Jon Perry (perry(AT)globalnet.co.uk), Feb 23 2004
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EXTENSIONS
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Terms a(10) and a(11) from John W. Layman (layman(AT)math.vt.edu), Mar 10 2004
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