where c 0 = c ( 0 ) {\displaystyle \mathbf {c} _{0}=\mathbf {c} (0)} is the statevector at t = 0 {\displaystyle t=0} . Here the exponential of a matrix may be found from the series expansion. The matrix U ( t ) {\displaystyle \mathbf {U} (t)} is called the time evolution matrix (which comprises the matrix elements of the corresponding time evolution operator U ( t ) {\displaystyle U(t)} ).