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 A090931 Number of permutations of 1,...,2n with alternate odd/even entries that have no adjacent members when placed in a circle. 1
 0, 0, 0, 8, 140, 5940, 316708, 21536080 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 LINKS Table of n, a(n) for n=1..8. EXAMPLE 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] PROG (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)) } CROSSREFS Cf. A092254. Sequence in context: A185248 A228867 A224735 * A239757 A295242 A305763 Adjacent sequences: A090928 A090929 A090930 * A090932 A090933 A090934 KEYWORD nonn AUTHOR Jon Perry, Feb 26 2004 EXTENSIONS a(7)-a(8) from Alois P. Heinz, Nov 19 2013 STATUS approved

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Last modified June 9 09:54 EDT 2023. Contains 363178 sequences. (Running on oeis4.)