Notice:

Any interconnection of linear voltage sources, current sources, and ohmic resistors can be.

• as a single, linear voltage source
  (Thévenin theorem) or
• as a single, linear current source
  (Norton theorem)

In Abbildung ## it can be seen that the three circuits give the same result (voltage / current) with the same load. This is also true when an (AC) source is used instead of the load.