Could you please give me an example of what you mean. I'm not quite sure that I understand your question.

If what you mean is that thebehaviorof the graph tends to infinity as x tends to a nuber c, we say that (does not exist), because thedefinition of limit states that a function f(x) has a limit as x tends to c only if, as x tends to c, f(x) tends to some number L.

Does that help?