Augmented equivalence principle algorithm at low frequencies

The present equivalence principle algorithm (EPA) is augmented by introducing charge densities as extra unknowns. This helps to separate the vector potential term and scalar potential term and avoid the imbalance at low frequencies. Current continuity constraint is enforced in both the scattering operator and translation operator. These further form a new augmented EPA equation system. With this technique, the low-frequency breakdown of EPA is removed. Besides, it serves not only as a stable low-frequency method but also as a substitute over the whole frequency band. The new scheme is verified by numerical examples.