1. Joseph Keller, Calculus of variations and its applications, Proc. of symposia in appl. math., vol. VIII,. pp. 27–52. McGraw-Hill, New York, 1958. Contains a bibliography of related work.
2. Hans Lewy, On the local character of the solutions of an atypical linear differential equation in three variables and a related theorem for regular functions of two complex variables, Ann. Math., vol. 64, #3 (1956), pp. 514–522.
3. Gaetano Fichera, Caratterizzazione della traccia, sulla frontiera di un campo, di una funzione analytica di più variabili complesse, Rend. Acc. Linc., serie VIII, vol. XXII, fasc. 6(1957), pp. 706–715.
4. Hans Lewy, An example of a smooth linear differential equation without solution, Ann. Math., vol. 66 #1(1957), pp. 155–158.
5. Philip Hartman, On smooth linear partial differential equations without solutions, Proc. Ani. Math. Soс., vol. 10, #1(1959),pp.252–257.