The modeling of systems by differential equations usually requires that the parameters involved be completely known. Such models often originate from problems in physics or economics where we have insufficient information on parameter values. For example, the values can vary in time or space due to unknown conditions of the surroundings or of the medium. In some cases the parameter values may depend in a complicated way on the microscopic properties of the medium. In addition, the parameter values may fluctuate due to some external or internal “noise”, which is random — or at least appears so to us.
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