Jan 27 2005 - 3:30pm
Adam Hammett
The Ohio State University
MW154
In combinatorics, much attention has been paid to properties of
the Bruhat order on $S_n$. We show that with high probability, two
permutations from $S_n$ selected independently and uniformly at random
will not even be comparable in the Bruhat order. In particular, for $\pi
,\sigma \in S_n$, we show $P[ \pi \leq \sigma ]=O( n^{-1/2})$, thus
obtaining an explicit rate of convergence to zero for $P[ \pi \leq \sigma
]$ as $n \rightarrow \infty$. This result is found via the well-known
Ehresmann's tableau criterion for comparability of permutations in the
Bruhat order and a connection to a certain random walk on the real line.
We also draw some conclusions about a lower bound for $P[ \pi \leq \sigma
]$, showing that $P[ \pi \leq \sigma ]=\Omega ( \alpha^n\right)$ for each
$\alpha < (1/2+(5/12)^{1/2})/2=0.5727\dots$, by utilizing a certain
recursive inequality.