Euler's invention of integral transforms / Michael A.B. Deakin.
Abstract:
"Euler invented integral transforms in the context of 2nd order differential equations...In introducing them he made use of earlier work in which a concept akin to the integral transform is explicit. It would, however, be reading too much into that earlier work to see it as contributing to the theory of the integral transform."
Archive for history of exact sciences Vol. 33 (1985), p. 307-319