adj. (of a first-order ordinary differential equation) such that, where u and t are respectively the dependent and independent variables, either it can be solved for p = du/dt and the resulting first-degree equation in p can be solved; or else it can be solved for u, giving an equation that can be differentiated to give a linear first-order equation in the dependent variable t and independent variable p; or else it can be solved for t, giving an equation that can be differentiated with respect to u to give a linear first-order equation in the dependent variable u and the independent variable p, after writing