Subsets of the set ω of
nonnegative integers which possess some algebraic structure are interesting since they
are most likely to give number-theoretic information. Arithmetic progressions
are one of the simplest structures to observe. Effectiveness of any kind of
information is of course an important factor. It seems that a study of possible
interrelationships between combinatoric and number-theoretic properties
of recursively enumerable (r.e.) subsets of ω might be interesting. In this
paper we study van der Waerden’s theorem on arithmetic progressions in this
light.