Substitution sequences in \mathbb{Z}^{d} with a non-simple Lebesgue component in the spectrum

Abstract

We construct d-dimensional substitution sequences for which the continuous part of the spectrum is generated by measures equal to Lebesgue measure. A special case is the Rudin–Shapiro substitution sequence. The construction uses Hadamard matrices in an essential way, so the dimension and size of a substitution is restricted by the size of the Hadamard matrix defining it. Each such substitution automatically has a dual substitution, which is defined by the same Hadamard matrix, and which retains a Lebesgue spectral component. We also see that random application of our substitutions produces sequences with a Lebesgue component in their spectrum. Finally, we see that any d-dimensional substitution with d > 1 can be ‘unraveled’ into lower-dimensional substitutions which still have Lebesgue spectral components.