Fraenkel's problem:
A word is balanced if the number of occurrences of any letter in any two factors of equal length differ at most by 1. Given a
k-letter alphabet
A, with
k ≥ 3, is there a unique balanced infinite word with distinct letter frequencies over
A? It is known to be true for
k ≤ 8.
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JeffreyShallit - 13 Jul 2011