# Problem 85: Short Square and Local Period

The notion of local periods in words provides a more accurate structure of its repetitiveness than its global period. The notion is central to that of critical positions (see Problem 41) and their applications.

Finding the local period at a given position $i$ on a word $x$ is the question of the problem. The local period $\lrep(i)$ is the period of a shortest non-empty square $ww$ centred at position $i$ and possibly overflowing $x$ to the left or to the right (or both). Show how to compute all non-empty squares centred at a given position $i$ on $x$ in time $O(|x|)$.

If there exists a shortest non-empty square of period $p$ centred at position $i$ on $x$, show how to find it in time $O(p)$.
Double the length of the search area.

Design an algorithm to compute the local period $p$ at position $i$ on $x$ in time $O(p)$.
Mind the situation where there is no square centred at $i$.

## References

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