His interests in other sciences, including history of mathematics, philosophy of science and computer science, grew as he continued to publish his mathematical work. Moderne mathematische Methoden in der Technik Amongst his contributions, Fenyő was mainly successful for publishing three encyclopedic volumes of his textbook, "Moderne mathematische Methoden in der Technik" that involve classical analysis, geometry and algebra. The first volume includes set theory, Lebesgue and Stieltjies integrals, calculus and differential equations. Fenyő, along with his coauthors, proved Titchmarsh's theorem, which is important to integral theory. Unlike the first and the third volumes, the second contains "a mixture of topics", like linear algebra, graph theory and network theory, that are used in engineering and technology. The third volume involves integral equations and functional analysis that deal "with the theory of operators". Integral equations One of Fenyő's main interests was integral equations. In 1976, he wrote "Über die Wiener-Hopfsche Integralgleichung"; it focuses on the nature of the set of solutions of the Wiener-Hopf integral equation for the case "where and are permitted to be tempered distributions". "Theorie und Praxis der linearen Integralgleichungen" was the six-volume work, written by H-W Stolle and Fenyő, that made significant contributions to integral equations. The first volume "is devoted to the theory of linear operators", and the second volume discusses the theory of integral equations "of the second kind". In the third volume, Fenyő explores the applications of integral transforms to mathematical physic and types of integral equations. Based on A E Heins' review of the three final volumes, those volumes focus on classical theory of linear integral equations that helped the "development of integral equations". Functional equations Fenyő also made a huge number of contributions to functional equations. One of his works, "The solution of a functional equation by Laplace transformation", focuses on proving two theorems that the functional equation has an analytic solution. He also discovered the "most general solution " of the following functional equation: In 1980's, he proved the theorem, using D.H. Hyers' propositions for solutions of functional equations. Fenyő, along with Gian Luigi Forti, also found the solutions to the following inhomogeneous Cauchy functional equation in a Banach space : where and is a bounded function. He was also known for discovering types of Jacobian functions that are related to functional equations. In the late 1980s, he collaborated with Paganoni in discovering a rule of rational addition in functional equation. The