@dmm symbolic operators: i.e. "p" (or "s" if you will)) "Treatises describing the application of operator methods to ordinary and partial differential equations were written by Robert Bell Carmichael in 1855[3] and by Boole in 1859.[4] This technique was fully developed by the physicist Oliver Heaviside in 1893, in connection with his work in telegraphy. Guided greatly by intuition and his wealth of knowledge on the physics behind his circuit studies, [Heaviside] developed the operational calculus now ascribed to his name." He was also willing to work with 𝑝^1/3 :) Quote from https://en.wikipedia.org/wiki/Operational_calculus I can't imagine the electronic filter theory developed from 1920--xxx without his operators.