Hamming Distance for Conjugates

Let x, y be strings of equal length. The Hamming distance h(x,y) between x and y is the number of positions in which x and y differ. If x is a cyclic shift of y, we say x and y are conjugates. We consider f(x,y), the Hamming distance between the conjugates xy and yx. Over a binary alphabet f(x,y) is always even, and must satisfy a further technical condition. By contrast, over an alphabet of size 3 or greater, f(x,y) can take any value between 0 and |x|+|y|, except 1; furthermore, we can always assume that the smaller string has only one type of letter.Comment: revisio

Polynomial versus Exponential Growth in Repetition-Free Binary Words

It is known that the number of overlap-free binary words of length n grows polynomially, while the number of cubefree binary words grows exponentially. We show that the dividing line between polynomial and exponential growth is 7/3. More precisely, there are only polynomially many binary words of length n that avoid 7/3-powers, but there are exponentially many binary words of length n that avoid (7/3+)-powers. This answers an open question of Kobayashi from 1986.Comment: 12 page