1. R. A. ADAMS: Jal of Diff. Eq., vol 9, no 9, 1971.
2. F. BROWDER: CRAS, t. 236, p. 214, 1953.
3. C. CLARK: Pac Jal Math. 19, 1964.
4. R. COURANT D. HILBERT: Methods of mathematical physics.
5. J. FLECKINGER: CRAS, t. 288, 7 mai 1979, série A, p. 801.