Linearity
Given 2 functions x1 and x2, constants a and b, outputs y1 and y2, and system s A system is linear if you take x1->S->y1, x2->S->y2 z=a*y1+b*y2 a*x1+b*x2->S->w If z==w then the system is linear If z!=w then the system is nonlinear This is better demonstrated by the diagram and examples below.
