A lot of mathematical knowledge has been formalized and stored in repositories by now: Different mathematical theorems and theories have been taken into consideration and included in mathematical repositories. Applications more distant from pure mathematics, however -- though based on these theories -- often need more detailed knowledge about the underlying theories. In this paper we present an example Mizar formalization from the area of electrical engineering focusing on stability theory which is based on complex analysis. We discuss what kind of special knowledge is necessary and which amount of this knowledge is included in existing repositories.