for partial differential equations [79] and in 1893 for ordinary differential equations [80], he
introduced monotone iterations from a lower solution. This is the starting point of the use of
lower and upper solutions in connection with monotone methods.Independently, some of the
basic ideas of the method appeared in the study of first order Cauchy problems made in
1915 by Perron [78] and in its extension to systems worked by Müller [72] in 1926. These …