Strictly Increasing Infinite Sequence of Positive Integers is Cofinal in Natural Numbers
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Definition
Let $S = \sequence {x_n}$ be an infinite sequence of positive integers which is strictly increasing.
Then $S$ is a cofinal subset of $\struct {\N, \le}$ where $\le$ is the usual ordering on the natural numbers.
Proof
By definition, $S = \sequence {x_n}$ is a subset of $\N$.
The result follows from Subset of Natural Numbers is Cofinal iff Infinite.
$\blacksquare$