(theory of operators satisfying the property of superposition)

Linear Theory, i.e., the study of Linear Systems
concerns operators (functions) that satisfy the
property of Superposition, i.e., if function
F is such that A = F(x) and B = F(y), then
A + B = F(x+y).

Such a system has nice properties when the
Fourier Transform is applied: the sum of
sine waves according to the transform's output
matches the input.

Linear Theory is used in signal processing and
analysis of time-variant functions.