On the M\"obius Function of Permutations With One Descent

The set of all permutations, ordered by pattern containment, is a poset. We give a formula for the M"obius function of intervals $[1,\pi ]$ in this poset, for any permutation $\pi$ with at most one descent. We compute the M"obius function as a function of the number and positions of pairs of consecutive letters in $\pi$ that are consecutive in value. As a result of this we show that the M"obius function is unbounded on the poset of all permutations. We show that the M"obius function is zero on any interval $[1,\pi ]$ where $\pi$ has a triple of consecutive letters whose values are consecutive and monotone. We also conjecture values of the M"obius function on some other intervals of permutations with at most one descent.