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24m

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Complexity :: Integer Programming :: NonPoly Example
the notions of NPcompleteness and polynomialtime solvability are about classes of problems, and not individual examples. 
10h

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Mysterious identity between numbers of odd/even meander systems
I bet this is a relatively easy exercise with generating functions; a one Richard Stanley can do in his head :) 
10h

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Mysterious identity between numbers of odd/even meander systems
So, do you have explicit formulae for $OMS$ and $EMS$, or not? 
10h

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Mysterious identity between numbers of odd/even meander systems
then what kind of "explicit expressions" are you talking about? 
10h

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Mysterious identity between numbers of odd/even meander systems
Then why do you ask "whether this is true"? So the numbers are the same, as you just said. Do you rather ask for an explicit bijection? 
10h

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Mysterious identity between numbers of odd/even meander systems
Did you try putting your numbers into oeis.org and see what it tells you? 
1d

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Is anything known about the eigenspectrum of the regular representation of the permutation group?
@QiaochuYuan  right, I missed this point. 
1d

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Is anything known about the eigenspectrum of the regular representation of the permutation group?
@QiaochuYuan  the behaviour of a permutation matrix is not determined by the order alone; it is determined by the cyclic structure of the permutation. E.g. the multiplicity of eigenvalue 1 is the number of cycles. 
2d

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Is anything known about the eigenspectrum of the regular representation of the permutation group?
a similar argument works for elements of order 3; as an eigenvalue $\lambda$ and its conjugate must occur with the same multiplicity, we see that the number of eigenvalues equal to 1 must be equal to twice the number of eigenvalues $\zeta$, for $\zeta$ a fixed primitive cubic root of unity. 
2d

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Is anything known about the eigenspectrum of the regular representation of the permutation group?
for the elements of order 2, it's trivial that the number of eigenvalues equal to 1 equals the number of eigenvalues equal to 1, as the trace of each nonidentity element equals to 0. 
2d

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Is anything known about the eigenspectrum of the regular representation of the permutation group?
@Anirbit: ARupinski claims something for the regular representation (of any finite group) only; Stembridge gives formulae for the irreducible representations of $S_n$, which is a completely different story. 
2d

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Is anything known about the eigenspectrum of the regular representation of the permutation group?
there is no contradiction; well, I don't see immediately how to (dis)prove ARupinski's statement, but it was probably already known to Frobenuis... 
Apr 24 
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A digraph related to permutations
takes 0.5 sec on my laptop (n is the number after $6!^{720}$) 
Apr 24 
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A digraph related to permutations
sage: factor(n) 2^307 * 3^120 * 5^19 * 7^128 * 11 * 13^3 * 19^44 * 31 * 43^44 * 47 * 61 * 113^6 * 127 * 131 * 139^2 * 241 * 1031 * 1481^2 * 1531 * 1621^2 * 1699^2 * 2801^3 * 5147 * 10607 * 47917^2 * 90803^2 * 548521 * 685367 * 2460593 * 47389957 * 2920679263^2 * 8428156447^2 * 216271121699 * 285106814092712039^2 
Apr 23 
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Ordered lattice point enumeration
@SteveHuntsman  LattE will only count the points 
Apr 23 
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A digraph related to permutations
actually, I wonder if BEST really applies, as $P_n$ has degree $n$, whereas for BEST one would want degree $n+1$, at least naively. 
Apr 22 
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Synthetic projective definition of cubic curves
@MikeShulman  this is some particular kind of divisor, IIRC... 
Apr 22 
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A digraph related to permutations
@DavidFeldman, sure, you're welcome! 
Apr 22 
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A digraph related to permutations
then this asks for en.wikipedia.org/wiki/BEST_theorem 
Apr 22 
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A digraph related to permutations
if you consider en.wikipedia.org/wiki/De_Bruijn_graph, more precisely the one on $n^n$ vertices, then your graph is a subgraph where each vertex corresponds to a permutation of $n$ symbols, right? 