A linear differential equation is a differential equation in which the dependent variable and its derivatives appear only in a linear form. It can be written in the form:
where y is the dependent variable, y' represents its derivative with respect to x, a_n(x), a_{n-1}(x), ..., a_1(x), a_0(x) are functions of x, and g(x) is a given function of x. The highest power of y and its derivatives in the equation is n, and the coefficients a_n(x), a_{n-1}(x), ..., a_1(x), a_0(x) can be constants or functions of x.