@article{1183739784,
author = {Benthem, J. F. A. K. Van},
title = {Modal Formulas are Either Elementary or not $\Sigma\Delta$-Elementary},
journal = {J. Symbolic Logic},
volume = {41},
number = {1},
year = {1976},
pages = { 436-438},
language = {en},
url = {http://dml.mathdoc.fr/item/1183739784}
}
Benthem, J. F. A. K. Van. Modal Formulas are Either Elementary or not $\Sigma\Delta$-Elementary. J. Symbolic Logic, Tome 41 (1976) no. 1, pp. 436-438. http://gdmltest.u-ga.fr/item/1183739784/