2.3 references

1. An elementary treatise on differential equations. By Abraham Cohen. 1906.
2. Applied differential equations, N Curle. 1972
3. Ordinary differential equations, LB Jones. 1976.
4. Elementary differential equations, William Martin, Eric Reissner. second edition. 1961.
5. Differentialgleichungen, by E. Kamke, page 30.
6. Differential and integral calculus by N. Piskunov, Vol II

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