Reciprocal Sequence is Strictly Decreasing

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Theorem

The reciprocal sequence:

$\sequence {\operatorname {recip} }: \N_{>0} \to \R$: $n \mapsto \dfrac 1 n$

is strictly decreasing.


Proof 1

Follows from Reciprocal Function is Strictly Decreasing and from Restriction of Monotone Function is Monotone.

$\blacksquare$


Proof 2

Let $n \in \N_{>0}$.

\(\displaystyle \frac 1 n - \frac 1 {n + 1}\) \(=\) \(\displaystyle \frac {\paren {n + 1} - n} {n \paren {n + 1} }\)
\(\displaystyle \) \(=\) \(\displaystyle \frac 1 {n^2 + n}\)
\(\displaystyle \) \(>\) \(\displaystyle 0\)
\(\displaystyle \leadsto \ \ \) \(\displaystyle \frac 1 n\) \(>\) \(\displaystyle \frac 1 {n + 1}\)

Hence the result, as $n$ was arbitrary.

$\blacksquare$


Also see