Ernst D. Hellinger joined the faculty at Northwestern as lecturer in Mathematics in 1939. He was promoted to professor in 1945, and retired in 1949. He studied integral equations, infinite systems of equations, real functions, and continued fractions. A type of integral he introduced in his dissertation became known as "the Hellinger integral," and the Hilbert-Hellinger theory of forms in infinitely many variables profoundly influenced mathematical analysis. His papers fill one and one-half...