In control theory, we may need to find out whether or not a system such as x ˙ ( t ) = A x ( t ) + B u ( t ) y ( t ) = C x ( t ) + D u ( t ) {\displaystyle {\begin{aligned}{\dot {\boldsymbol {x}}}(t)&={\boldsymbol {Ax}}(t)+{\boldsymbol {Bu}}(t)\\{\boldsymbol {y}}(t)&={\boldsymbol {Cx}}(t)+{\boldsymbol {Du}}(t)\end{aligned}}} is controllable, where A {\displaystyle {\boldsymbol {A}}} , B {\displaystyle {\boldsymbol {B}}} , C {\displaystyle {\boldsymbol {C}}} and D {\displaystyle {\boldsymbol {D}}} are, respectively, n × n {\displaystyle n\times n} , n × p {\displaystyle n\times p} , q × n {\displaystyle q\times n} and q × p {\displaystyle q\times p} matrices for a system with p {\displaystyle p} inputs, n {\displaystyle n} state variables and q {\displaystyle q} outputs.
One of the many ways one can achieve such goal is by the use of the Controllability Gramian.