Courant 1937i, II, §9: "Here we remark merely in passing that it is possible to use this approximate representation of the increment Δy by the linear expression hƒ(x) to construct a logically satisfactory definition of a "differential", as was done by Cauchy in particular." Courant, Richard (1937i), Differential and integral calculus. Vol. I, Wiley Classics Library, New York: யோன் வில்லி அன் சன்ஸ் (published 1988), பன்னாட்டுத் தரப்புத்தக எண்978-0-471-60842-4, MR1009558.