Source: know almost
The variables in Equations set x (may represent a plurality of variables) to be demand function is y = y (x).
Homogeneous
Differential does not contain a constant term, nor only composed of various combinations of operations of the x term (such as 4xx, sinx, etc.);
Each must contain y base and derived classes (a pit can have multiple y family members, they can be mixed together just to play, such as what to enhance their power)
Linear
Various arithmetic combinations of a mixed terms of differential equations containing only power of y and its first derivative, or an x term and the first power of the configuration,
contain higher powers of y, and the number of items of various orders, nor with a mixture term between y and its first derivative (containing only ay, by ', xy "term for a class of free ayy, byy', cxyy" a class of item. (abc is a constant))
Each square must contain a time base and derived classes of y (a pit only members of a family of y, with the body and must be honest "primary power" stay)