an equation (= a mathematical statement) that expresses a relationship between a function (= a changing mathematical value) and two or more of its derivatives (= measures of the rate at which the function is changing) in which one variable (= value) changes and others stay the same

This has been an elusive goal for developers of partial differential equation solving software. 这一直是偏微分方程求解软件开发人员难以实现的目标。