Equation of longitudinal vibrations of a beam with coefficients periodically depending on the coordinate

The equation of small longitudinal vibrations of a beam is considered for the case in which the coefficients are periodic and have “gaps” (sharp decreases in the value) at some point in the space inside the periodicity interval. We suggest to describe the process locally in the domain of minimum of the coefficients by an equation of the boundary layer approximation. Attention is mainly paid to the analysis of this equation. A time hierarchy of simplified models for describing this process is established.