Thursday, November 13, 2014 - 9:00am - 9:50am
Ira Gessel (Brandeis University)
We count permutations in which every valley is in an even position and every peak is in an odd position. Following Dennis Chebikin, we say that a position i of a permutation p is an “alternating descent” if i is odd and a descent of p, or if i is even and an ascent of p, and we define alternating runs to be maximal consecutive subsequences with no alternating descents. Then the permutations to be counted are those with no alternating runs of length 3 or more.
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