A word y is said to be a "factor" of a word w if there exist possibly empty words x, z such that w = xyz. For example, "art" is a factor of "Sparta".
Caution should be exercised, since what in Europe is called "factor" is often called "subword" in North America. See, for example, the entry for
subword complexity.
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JeffreyShallit - 13 Jul 2011