I was very close to Chris during 1975-1990. He visited Groningen and I visited him in Arizona and at Washington University for several extended periods. We worked together on a number problems during these times. Our cooperation in the area of adaptive control and nonlinear dissipativity led to several articles of which I think back with pride and satisfaction.

Collaborating with Chris were memorable experiences. He was eager to share his ideas and his mathematical culture. His exceptional quickness and his marvelous sense of humor made working with Chris both efficient and pleasant. I admired his ability to keep concentrated on scientific questions while multitasking with administrative details that he needed to worry about simultaneously. He was always able to find a common ground in our interests and I found him to be a good and critical listener. I learned so much from him, also on the historical origin of mathematical ideas.

Chris and Renee stayed at our home in Antwerp in 2008 for a few days. They were on their way from Amsterdam to London. He told me that he was back to full time research and full of ideas. He explained what he was going to tell at his coming Bode lecture in Cancun. We made plans to restart our collaboration. Unfortunately, this was delayed because a few months later I became ill. The conference which I attended in honor of Chris and Anders in September 2009 in Stockholm was my first trip after my recovery. I was delighted that Chris had found a set-up at KTH which would allow him to concentrate on research.

Unfortunately, fate decided otherwise. I was deeply saddened when I learned of his passing away. With Chris, I lost a congenial colleague and collaborator for over 35 years, and a very good friend. I will miss his wit, his wisdom, and his warmth.