S. Izumiya, THE THEORY OF LEGENDRIAN UNFOLDINGS AND 1ST-ORDER DIFFERENTIAL-EQUATIONS, Proceedings of the Royal Society of Edinburgh. Section A. Mathematics, 123, 1993, pp. 517-532
We consider some properties of completely integrable first-order diffe
rential equations for real-valued functions. In order to study this su
bject, we introduce the theory of Legendrian unfoldings. We give a cha
racterisation of equations with classical complete solutions in terms
of Legendrian unfoldings, and also assert that the set of equations wi
th singular solutions is an open set in the space of completely integr
able equations even though such a set is thin in the space of all equa
tions.