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 A178217 Number of unsigned permutations in S_{3n-1} whose breakpoint graph contains only cycles of length 3. 0
 1, 12, 464, 38720, 5678400, 1294720000, 423809075200, 188422340198400, 109244157102080000, 80068011114291200000, 72384558633074688000000, 79125533869852634644480000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The number of permutations in S_{n} whose breakpoint graph contains only cycles of length 3 is nonzero only for n=3*k-1 (see references for definitions). LINKS J.-P. Doignon and A. Labarre, On Hultman Numbers, J. Integer Seq., 10 (2007), 13 pages. A. Labarre, Combinatorial aspects of genome rearrangements and haplotype networks (2008), Ph. D. thesis. FORMULA a(n) = (3n)!/(n!*12^n)*Sum_{i=0..n} binomial(n,i)*3^i)/(2i+1). (See references for a proof.) EXAMPLE See references for examples (nongraphical explanations do not help much). PROG (Maxima) a(p) := ((3*p)!/(p!*12^p))*sum(binomial(p, i)*(3^i)/(2*i+1), i, 0, p); (PARI) a(n) = (3*n)!/(n!*12^n) * sum(i = 0, n, binomial(n, i)*3^i/(2*i+1)); \\ Michel Marcus, Sep 05 2013 CROSSREFS Sequence in context: A221496 A302945 A089956 * A262584 A241226 A003749 Adjacent sequences:  A178214 A178215 A178216 * A178218 A178219 A178220 KEYWORD nonn AUTHOR Anthony Labarre, Dec 25 2010 STATUS approved

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Last modified June 6 10:12 EDT 2020. Contains 334859 sequences. (Running on oeis4.)