#
Problem 86: The Number of Runs

A run is a maximal periodicity occurring in a word.
Formally, a run in $x$ is an interval $[i\dd j]$ of positions on $x$
whose associated factor $x[i\dd j]$ is periodic (i.e., its smallest
period $p$ satisfies $2p \leq |x[i\dd j]|=(j-i+1)$) and the
periodicity does not extend to the right nor to the left
(i.e., $x[i-1\dd j]$ and $x[i\dd j+1]$ have larger periods when
defined).

We consider an ordering $\lt$ on the word alphabet and the corresponding
lexicographic ordering denoted $\lt$ as well.
We also consider the lexicographic ordering $\widetilde{\lt}$, called
the reverse ordering, inferred by the inverse alphabet ordering
$\lt^{-1}$.
Each run $[i\dd j]$ is associated with its greatest suffix according
to one of the two orderings as follows.
Let $p=\per(x[i\dd j])$.
If $j+1\lt n$ and $x[j+1] \gt x[j-p+1]$ we
assign to the run the position $k$ for which $x[k\dd j]$ is
the greatest proper suffix of $x[i\dd j]$ according to $\lt$.
Otherwise, $k$ is the starting position of the greatest proper suffix
of $x[i\dd j]$ according to $\widetilde{\lt}$.
The position $k$ assigned this way to a run is called its **special
position**.
These positions are intimately linked to Lyndon words (defined in
Section Ordering), subject of the first
question.

Show that, if the special position $k$ of a run of period $p$ is defined
according to $\widetilde{\lt}$ (resp. $\lt$), $x[k\dd k+p-1]$ is the longest
Lyndon factor of $x$ starting at position $k$ according to $\lt$ (resp.
$\widetilde{\lt}$).

The special position $k$ of a run $[i\dd j]$ of period $p$ satisfies
$k\leq i+p$; see

Problem 40.

Show two distinct runs have no special position in common and deduce that the
number of runs in a word is smaller than its length.

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