  We briefly review a matrix based method to compute the Casimir operators of
Lie algebras, mainly certain type of contractions of simple Lie algebras. The
versatility of the method is illustrated by constructing matrices whose
characteristic polynomials provide the invariants of the kinematical algebras
in (3+1)-dimensions. Moreover it is shown, also for kinematical algebras, how
some reductions on these matrices are useful for determining the missing
operators in the missing label problem (MLP).
