%0 Journal Article
%A A. V. Chagrov
%T Undecidable properties of extensions of provability logic.~II
%J Algebra i logika
%D 1990
%P 613-623
%V 29
%N 5
%I mathdoc
%U http://geodesic.mathdoc.fr/item/AL_1990_29_5_a6/
%G ru
%F AL_1990_29_5_a6