References
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- Spiegel , M. R. 1983 . Schaum's Outline of Theory and Problems of Advanced Mathematics for Engineers and Scientists , New York : McGraw‐Hill .
- Friedrichs , K. O. 1967 . Lectures on Advanced Ordinary Differential Equations , London : Nelson .
- Schwabik , S. 1993 . Differential and Integral Equations, , 6 : 863 – 882 .