Let a function f(x , y ) of the two real variables x and y have domain of defini-
tion D in which there lies the point Q at (x0, y0), and let L be a real number. In
addition, let g be any path (curve in the (x , y)-plane) joining an arbitrary point
P in D to the point Q at (x0, y0). We shall say a point (x , y ) on g approaches
arbitrarily closely to the point (x0, y0), written either x0/x0, y0/y0 or (x , y)0/ (x0, y0), if the distance from (x , y) to (x0, y0) becomes arbitrarily small.