Longest increasing subsequences and log concavity

الأصل · EN

Let π be a permutation of [n]={1,,n} and denote by ℓ(π) the length of a longest increasing subsequence of π. Let ℓₙ,ₖ be the number of permutations π of [n] with ℓ(π)=k. Chen conjectured that the sequence ℓₙ,₁,ℓₙ,₂,,ℓₙ,ₙ is log concave for every fixed positive integer n. We conjecture that the same is true if one is restricted to considering involutions and we show that these two conjectures are closely related. We also prove various analogues of these conjectures concerning permutations whose output tableaux under the Robinson-Schensted algorithm have certain shapes. In addition, we present a proof of Deift that part of the limiting distribution is log concave. Various other conjectures are discussed.