Sure, I was able to find some information about Hans Sagan, the author of the book "Introduction to the Calculus of Variations". According to the "History of the Mathematics Department at North Carolina State University" by Nicholas J. Rose, Hans Sagan obtained a PhD from the University of Vienna in 1950. His dissertation was on the minimum of a double integral.

Sagan wrote nine books, including "Space-Filling Curves" and "Boundary and Eigenvalue Problems in Mathematical Physics". He also published more than twenty papers on mathematics. Sagan was a full professor at North Carolina State University and had six PhD students. He retired in 1994.

I hope this helps! If you have any other questions about the authors of the textbooks I mentioned, or about anything else, I'd be happy to assist.
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