We prove that there exists a structure whose monadic second order theory is decidable, and such that the first-order theory of every expansion of by a constant is undecidable.
@article{ITA_2008__42_1_137_0, author = {B\`es, Alexis and C\'egielski, Patrick}, title = {Weakly maximal decidable structures}, journal = {RAIRO - Theoretical Informatics and Applications - Informatique Th\'eorique et Applications}, volume = {42}, year = {2008}, pages = {137-145}, doi = {10.1051/ita:2007044}, mrnumber = {2382548}, zbl = {1149.03015}, language = {en}, url = {http://dml.mathdoc.fr/item/ITA_2008__42_1_137_0} }
Bès, Alexis; Cégielski, Patrick. Weakly maximal decidable structures. RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications, Tome 42 (2008) pp. 137-145. doi : 10.1051/ita:2007044. http://gdmltest.u-ga.fr/item/ITA_2008__42_1_137_0/
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