The limit would in fact be 6 even if f ðx; yÞ were not defined at
ð1; 2Þ. Thus the existence of the limit of f ðx; yÞ as ðx; yÞ ! ðx0; y0Þ is in no way dependent on the existence of a value
of f ðx; yÞ at ðx0; y0Þ.
Note that in order for lim
ðx;yÞ!ðx0;y0Þ
f ðx; yÞ to exist, it must have the same value regardless of the
approach of ðx; yÞ to ðx0; y0Þ. It follows that if two different approaches give different values, the
limit cannot exist (see Problem 6.7). This implies, as in the case of functions of one variable, that if a
limit exists it is unique.
The concept of one-sided limits for functions of one variable is easily extended to functions of more
than one variable.