Let k and l be two multiplicatively independent integers, and let L. s
ubset of or equal to N-n be a l-recognizable set which is not definabl
e in [N; +]. we prove that the elementary theory of [N; +, V-k, L], wh
ere V-k(x) denotes the greatest power of k dividing x, is undecidable.
This result leads to a new proof of the Cobham-Semenov theorem.