Informally, the term asymptotic means approaching a value or curve arbitrarily closely (i.e., as some sort of limit is taken). A line or curve that is asymptotic to given curve is called the asymptote of .
More formally, let be a continuous variable tending to some limit. Then a real function and positive function are said to be asymptotically equivalent, written , if
(1)
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as the limit is taken.
Equivalently, consider the little-o asymptotic notation that is one of the Landau symbols. Then means that
(2)
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as a limit is taken. The statement is then equivalent to
(3)
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or
(4)
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(Hardy and Wright 1979, pp. 7-8).
These definitions can also be applied to the discrete case of an integer variable that tends to infinity, a real function of , and a positive function of .