On the Sum of the Heights of Sturmian Factors
  A binary word is a map W : N --> {0,1}, and the set of factors of W with length n is F_n(W):={(W(i),W(i+1),...,W(i+n-1)) : i >= 0}. A word is Sturmian if |F_n(W)|=n+1 for every n>0. We show that the sum of the heights (also known as hamming weights) of the n+1 factors with length n of a binary Sturmian word has the same parity as n, independent of W. 
Kevin O'Bryant