On the Asymptotic Expansions for Time-Varying Scalar Differential   Equations Possesssing Limiting Differential Equations by Application of the   Residue Theorem to their Discretized Counterparts. Preliminary Results
  In this paper, it is proved that, in a dual context, asymptotic expansions of ordinary linear time-differential equations which possess limiting equations to their limiting equations might be obtained by first discretizing them and then using residues calculation for such discrete- time counterparts 
M. De la Sen