Uber eine Klasse singularer Integralgleichungen. Offprint from: Sitzungsberichten der Preussischen Akademie der Wissenschaften. Phys.-Math. Klasse, Vol. XXXI

Berlin: Akademie der Wissenschaften, 1931. FIRST EDITION. The author's original offprint in the original printed wrappers. First announcement of the Wiener-Hopf equation, or the integral equation for (x). This equation occurs commonly in many applications. In its solution a study of the Fourier transform in the complex plane is necessary. The Fourier transform as a function of a complex variable proved to be very important in Wiener's subsequent research.
Wiener's interest in the integral equation led to a second collaboration with Hopf on the investigation of the equations for radiation equilibrium in the stars.