Continuity of f at a means
lim
( )
x
a f x
→+
= f (a)
and continuity of f  at b means
lim–
( )
x
b f x
→
= f(b)
Observe that lim
( )
x
a f x
→−
 and lim
( )
x
b f x
→+
do not make sense As a consequence
of this definition, if f is defined only at one point, it is continuous there, i e , if the
domain of f is a singleton, f is a continuous function