Theory of differences and sums of quantities
Definitions and preliminary notions
(1.) A function of a given variable is defined as any arithmetic expression involving this variable, irrespective of how it appears in it.
Thus x, a + bx, (c r etc. are functions of x.
Consider X an arbitrary function of x, and define as what becomes of X when x is replaced by x+k; then represents the variation of X when x increases by k. is called