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A090931
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Number of permutations of 1,..,2n with alternate odd/even entries that have no adjacent members when placed in a circle.
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0
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OFFSET
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1,4
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LINKS
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Table of n, a(n) for n=1..6.
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EXAMPLE
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The 8 for a(4) are;
[7, 4, 1, 6, 3, 8, 5, 2]
[5, 2, 7, 4, 1, 6, 3, 8]
[1, 4, 7, 2, 5, 8, 3, 6]
[3, 8, 5, 2, 7, 4, 1, 6]
[3, 6, 1, 4, 7, 2, 5, 8]
[5, 8, 3, 6, 1, 4, 7, 2]
[1, 6, 3, 8, 5, 2, 7, 4]
[7, 2, 5, 8, 3, 6, 1, 4]
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PROG
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(PARI) { for (i=1, 6, c=0; for (j=1, i!, for (k=1, i!, x=numtoperm(i, j); y=numtoperm(i, k); v=vector(2*i); forstep (z=1, 2*i, 2, v[z]=x[(z+1)/2]*2-1); forstep (z=2, 2*i, 2, v[z]=y[z/2]*2); fl=1; for (z=1, 2*i-1, if (v[z]==v[z+1]+1, fl=0; break); if (v[z]==v[z+1]-1, fl=0; break)); if (v[2*i]==v[1]+1, fl=0); if (v[2*i]==v[1]-1, fl=0); if (fl==1, c++))); print1(", "c)) }
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CROSSREFS
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Cf. A078092.
Sequence in context: A212442 A185248 A224735 * A180357 A112464 A212703
Adjacent sequences: A090928 A090929 A090930 * A090932 A090933 A090934
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KEYWORD
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nonn
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AUTHOR
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Jon Perry, Feb 26 2004
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STATUS
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approved
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