This kind of problem is regular. . . When E [1/(X-A)] is encountered, the limit should be obtained from the positive and negative directions of a respectively.

Here we need to find the limit from the+-direction of 0. If they are equal, then there is an answer. If not, then the limit does not exist.

lim(x->； 0+)f(x)=- 1

lim(x->； 0-)f(x)= 1

The restriction does not exist.