Characterization of circular D0L systems
  We prove that every non-circular D0L system contains arbitrarily long repetitions. This result was already published in 1993 by Mignosi and S\'e\'ebold, however their proof is only a sketch. We give here a complete proof. Further, employing our previous result, we give a simple algorithm to test circularity of an injective D0L system. 
Karel Klouda and \v{S}t\v{e}p\'an Starosta
math.CO