a1 Aristotle University of Thessaloniki, Department of Mathematics, 54124 Thessaloniki, Greece; firstname.lastname@example.org
a2 Democritus University of Thrace, Department of Production Engineering and Management, 67100 Xanti, Greece; email@example.com
a3 Technical Institute of Kavala, Department of Exact Sciences, 65404 Kavala, Greece.
We display a complexity notion based on the syntax of a tree series which yields two distinct hierarchies, one within the class of recognizable tree series and another one in the class of non-recognizable tree series.
(Received March 17 2009)
(Accepted December 16 2009)
(Online publication January 26 2010)
Mathematics Subject Classification: