$x=a$ is a vertical asymptote of the graph of a function $f$ if for every $\mu \gt 0$ there exists $\delta \gt 0$ such that for all $x$ where $|x-a| \lt \delta$, this implies $|f(x)| \gt \mu$.
Is this a valid definition for vertical asymptote?
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1If you write "where $0 < |x-a|<\delta$" [to exclude input $x=a$] then it looks OK. – 2017-02-06