  It is known that any rational abstract numeration system is faithfully, and
effectively, represented by an N-rational series. A simple proof of this result
is given which yields a representation of this series which in turn allows a
simple computation of the value of words in this system and easy constructions
for the recognition of recognisable sets of numbers. It is also shown that
conversely it is decidable whether an N-rational series corresponds to a
rational abstract numeration system.
