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 A091217 Number of permutations p of [n] such that the n-1 sums p(i)+p(i+1) (i=1,2,...n-1) are all distinct. 2
 1, 2, 6, 16, 54, 232, 1132, 6024, 36262, 242080, 1775316, 14135584, 122077832, 1131066448, 11230979624, 118638940864 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS EXAMPLE 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. PROG (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)) } CROSSREFS Sequence in context: A147949 A147940 A147931 * A147891 A147922 A147913 Adjacent sequences:  A091214 A091215 A091216 * A091218 A091219 A091220 KEYWORD hard,more,nonn AUTHOR Jon Perry, Feb 23 2004 EXTENSIONS Terms a(10) and a(11) from John W. Layman, Mar 10 2004 More terms. R. H. Hardin, Nov 11 2008 Corrected definition. - Emeric Deutsch, Nov 28 2008 STATUS approved

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Last modified November 17 00:10 EST 2018. Contains 317275 sequences. (Running on oeis4.)