The sum of digits function of the base phi expansion of the natural numbers

Abstract: In the base phi expansion any natural number is written uniquely as a sum of powers of the golden mean with digits 0 and 1, where one requires that the product of two consecutive digits is always 0. In this paper we show that the sum of digits function modulo 2 of these expansions is a morphic sequence. In particular we prove that --- like for the Thue-Morse sequence --- the frequency of 0's and 1's in this sequence is equal to 1/2.