Kalman's Last Decade: Passive Network Synthesis [Historical Perspectives]

The Art Nouveau splendor of Budapest's Hotel Gellert was the scene for a fascinating day of scientific discourse between an 80-year-old Rudolf Kalman and me on a warm Sunday in July 2010. The focal point of the discussions was the classical theory of passive network synthesis-a beautiful subject that reached its zenith around 1960, only to decline steadily thereafter as an active research interest, and yet a subject that still raises many deep and intriguing questions. Widening conversations and a special session at the Mathematical Theory of Networks and Systems conference in the following days led the late Uwe Helmke to organize and host a workshop on network synthesis in September of that year in Wurzburg (see Figures 1 and 2), which initiated an inspirational series of meetings that has alternated with Cambridge. What was the origin of Kalman's intense interest in this topic?