Kolakoski sequence: links between recurrence, symmetry and limit density

The Kolakoski sequence $S$ is the unique element of { 1 , 2 } ω starting with 1 and coinciding with its own run length encoding. We use the parity of the lengths of particular subclasses of initial words of S as a unifying tool to address the links between the main open questions – recurrence, mirror/reversal invariance and asymptotic density of digits. In particular we prove that recurrence implies reversal invariance, and give sufficient conditions which would imply that the density of 1s is 1 2 .