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2. R. Courant, K. O. Friedrichs, and H. Lewy,Über die partiellen Differenzengleichungen der mathematischen Physik, Math. Ann., 100 (1928), pp. 37–74.
3. R. Courant and D. Hilbert,Partial Differential Equations, Methods of Mathematical Physics, vols. I and II, Interscience, New York, 1962.
4. B. Epstein,Partial Differential Equations, McGraw-Hill, New York, 1962.
5. P. Jamet,Numerical Methods for Singular Linear Boundary Value Problems, Doctoral thesis, University of Wisconsin, Madison, Wisconsin, 1967.