This paper presents a symbolic test-function defined on a restricted interval for describing the electromagnetic interaction. An exponential function with the exponent represented by the inverse of a differential operator is put in correspondence with a test function defined on a restricted interval. The lateral limits of this function at the edge of this interval are either zero (when the variable is considered in closer proximity inside this interval) or infinity (when the variable is considered in closer proximity outside this interval). By substituting with a differential operator these edges (stable points) can be put in correspondence with the wave function of charged particles. In a similar manner, the maximum of this function (an unstable point) can be put in correspondence with an electromagnetic propagating wave. As a consequence, the electromagnetic interaction can be described by particles with eigenvalues of differential operators confined within a restricted interval (similar to wavelets presenting a non-zero amplitude on limited space-time intervals).