Irregular partially invariant solutions of rank 2 and defect 1 to equations of gas dynamics. Khabirov, S.V. — 2002 Sibirskij Matematicheskij Zhurnal
Classification of differential invariant submodels. Khabirov, S.V. — 2004 Sibirskij Matematicheskij Zhurnal
A property of the defining equations for the Lie algebra in the group classification problem for wave equations. Khabirov, S.V. — 2009 Sibirskij Matematicheskij Zhurnal