Applying, Canceling and Identifying the Magnetic Field Received and Emitted by a 2D Plan System

2D multipolar expansion is extended to account for externally created field and numerous relations are derived. The most general field created outside or inside a cylinder can be created by a current density flowing axially on the surface of the cylinder. Such a surface current can also cancel the field generated either inside by external sources or outside by internal sources. Knowing 1 field component on 2 coaxial cylinders allows both internally and externally created fields to be identified