%0 Journal Article
%A L. M. Lerman
%A Ya. L. Umanskii
%T Classification of four-dimensional integrable Hamiltonian systems and Poisson actions of~$\mathbb{R}^2$ in extended neighborhoods of simple singular points.~I
%J Sbornik. Mathematics
%D 1994
%P 511-542
%V 77
%N 2
%I mathdoc
%U http://geodesic.mathdoc.fr/item/SM_1994_77_2_a15/
%G en
%F SM_1994_77_2_a15