Definition:Reduced Equation of Linear ODE with Constant Coefficients/First Order

Definition
Consider the linear first order ODE with constant coefficients:
 * $(1): \quad \dfrac {\d y} {\d x} + a y = \map Q x$

The equation:
 * $\dfrac {\d y} {\d x} + a y = 0$

is the reduced equation of $(1)$.