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  "body": "\n## Summary\n\nCombinatorics on words studies finite and infinite sequences of letters from an alphabet, see the [[definition|N815PEPPFA]].\nTypical topics are repetitions such as squares and overlaps, periodicity, unavoidable patterns, and the number of distinct factors of a word.\nClassical objects of the field are the [[Thue–Morse sequence|7JZVT385PC]] and the [[de Bruijn sequences|GY8F3M39R8]].\nIt has links with algebra through the free [[monoid|0N6R902FG1]] of words, with number theory, and with the theory of formal languages.\n\n## Explanations\n\n### L1 human\n\nA word is just a string of letters, such as $abaab$.\nCombinatorics on words asks which patterns must appear in long words and which can be avoided.\nFor example, every word of length $4$ on two letters contains a block repeated twice in a row, such as $aa$ or $abab$.\nOn three letters, by contrast, there are infinite words with no such repeated block.\n",
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