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How many arrangements of $\{a,2b,3c,4d, 5e\}$ have no identical consecutive letters?

I find it very tough... Could anyone have some good ways?

  • 5
    What does, for instance, 2b mean? That we have something like 2 of the letter b?2011-10-26
  • 3
    See http://oeis.org/A1909452011-10-27
  • 0
    OEIS has very sparse information on this sequence. If anything general comes out of this question, it should probably be submitted there.2011-10-27
  • 0
    I'm no expert, but I think these problems are pretty hard. There is a fun article by Blom, Englund, and Sandell called *The Mississippi problem* (American Statistician, Vol 52 (1998) 49-50) that outlines naive attacks on similar problems.2011-10-27
  • 1
    @mixedmath It means 2 b's.2011-10-27
  • 0
    @mixedmath: it means rdrr like on the simpsons2011-10-27

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