Izobov N. A.
On the countability of the number of solutions of a two-dimensional linear Pfaff system with different characteristic sets
Ukr. Mat. Zh. - 2007. - 59, № 2. - pp. 172–189
We prove that a two-dimensional, completely integrable, linear Pfaff system has at most countably many solutions with all different characteristic sets.
Ukr. Mat. Zh. - 1972. - 24, № 3. - pp. 305—314