From the standpoint of an algebraist, words are nothing but ordered sequences of letters, each letter being a member of an allowable alphabet of such letters. Any sequence of letters is permitted, and he is often interested in deriving theorems about the existence or non-existence of certain letter-patterns.
Eckler, A. Ross
"The Mathematics of Words,"
Word Ways: Vol. 16
, Article 16.
Available at: https://digitalcommons.butler.edu/wordways/vol16/iss4/16