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**System** − An entity which acts on an input signal and transforms it into an output signal is called the *system*.

**Linear System** − A *linear system* is defined as a system for which the principle of superposition and the principle of homogeneity are valid.

The principle of superposition states that the response of the system to a weighted sum of input signals is equal to the corresponding weighted sum of the outputs of the system to each of the input signals.

Therefore, if an input signal x_{1}(t) produces an output signal y_{1}(t) and another
input signal x_{2}(t) produces an output y_{2}(t), then the system is said to be linear if,

$$\mathrm{T[ax_{1}(t)+bx_{2}(t)]=ay_{1}(t)+by_{2}(t)}$$

Where,a and b are constants.

Linear systems can be of the following two types −

Linear Time-Invariant [LTI] System

Linear Time-Variant [LTV] System

A system which is both linear and time-invariant is called the *linear timeinvariant system.* In other words, a system for which both the superposition principle and the homogeneity principle are valid and the input-output characteristics of the system do not change with time is called *linear time
invariant (LTI) system.*

Therefore, for an LTI system, the output for a delayed input [x(t -t_{0})] must
equal to the delayed output [y(t -t_{0})], i.e.,

**$$\mathrm{y(t,t_{0})=y(t-t_{0})}$$**

It means that if the input to the system is delayed by (t_{0}) units, then the
corresponding output will also be delayed by(t_{o}) units. Also, for a linear timeinvariant system, all the coefficients of the differential equation describing the
system are constants.

A system which is linear but time-variant is called the *linear time-variant
system.* In other words, a system for which the principle of superposition and homogeneity are valid but the input-output characteristics change with time is called the *linear time-variant (LTV) system.*

Therefore, for a linear time variant (LTV) system, the output of the system
corresponding to the delayed input signal [i.e.,x(t -t_{0})] is not equal to the
delayed output [y(t -t_{0})], i.e.,

**$$\mathrm{y(t,t_{0})\neq y(t-t_{0})}$$**

It means that, if the input to an LTV system is delayed by $(t_{0})$ units, then the corresponding output will not be delayed by exactly $(t_{0})$ units. Also, for a linear time variant (LTV) system at least one of the coefficients of the differential equation describing the system vary with time.

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