1. A. V. Chagrov & L. A. Chagrova (1995). Algorithmic Problems Concerning First-Order Definability of Modal Formulas on the Class of All Finite Frames. Studia Logica 55 (3):421 - 448.
    The main result is that is no effective algorithmic answer to the question:how to recognize whether arbitrary modal formula has a first-order equivalent on the class of finite frames. Besides, two known problems are solved: it is proved algorithmic undecidability of finite frame consequence between modal formulas; the difference between global and local variants of first-order definability of modal formulas on the class of transitive frames is shown.
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  2. Dick de Jongh & L. A. Chagrova (1995). The Decidability of Dependency in Intuitionistic Propositional Logi. Journal of Symbolic Logic 60 (2):498-504.
    A definition is given for formulae $A_1,\ldots,A_n$ in some theory $T$ which is formalized in a propositional calculus $S$ to be (in)dependent with respect to $S$. It is shown that, for intuitionistic propositional logic $\mathbf{IPC}$, dependency (with respect to $\mathbf{IPC}$ itself) is decidable. This is an almost immediate consequence of Pitts' uniform interpolation theorem for $\mathbf{IPC}$. A reasonably simple infinite sequence of $\mathbf{IPC}$-formulae $F_n(p, q)$ is given such that $\mathbf{IPC}$-formulae $A$ and $B$ are dependent if and only if at least (...)
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  3. L. A. Chagrova (1991). An Undecidable Problem in Correspondence Theory. Journal of Symbolic Logic 56 (4):1261-1272.
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