Sylvestre Lacroix

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The word linear [applied to differential equations] is improper; it is relative to Geometry, and in applying it to equations, one has in view a straight line in the equation of which the ordinate [= y coordinate] (or equally the abscissa [= x coordinate]) are found only to the first degree: thus one would not know how to regard linear equations such as dy + Py dx = Q dx, which belong most often to transcendental curves.
Calculus (1799)