### linear theory

(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.

(*mathematics*)
**Further reading:**

http://en.wikipedia.org/wiki/Linear_system

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