Reciprocal of Null Sequence

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Let $\left \langle {z_n} \right \rangle$ be a sequence in $\C$.

Let $\forall n \in \N: z_n > 0$.


$(1): \quad z_n \to 0$ as $n \to \infty$ iff $\left|{\dfrac 1 {z_n}}\right| \to \infty$ as $n \to \infty$
$(2): \quad z_n \to \infty$ as $n \to \infty$ iff $\left|{\dfrac 1 {z_n}}\right| \to 0$ as $n \to \infty$.

Real Numbers

If $\left \langle {x_n} \right \rangle$ is a sequence in $\R$, the same applies.


  • Suppose $z_n \to 0$ as $n \to \infty$.

Let $H > 0$.

So $H^{-1} > 0$.

Since $z_n \to 0$ as $n \to \infty$, $\exists N: \forall n > N: \left|{z_n}\right| < H^{-1}$.

That is, $\left|{\dfrac 1 {z_n}}\right| > H$.

So $\exists N: \forall n > N: \left|{\dfrac 1 {z_n}}\right| > H$ and thus $\left \langle {\left|{\dfrac 1 {z_n}}\right|} \right \rangle$ diverges to infinity.

  • Suppose $\left|{\dfrac 1 {z_n}}\right| \to \infty$ as $n \to \infty$.

By reversing the argument above, we see that $z_n \to 0$ as $n \to \infty$.


The statement:

$z_n \to \infty$ as $n \to \infty$ iff $\left|{\dfrac 1 {z_n}}\right| \to 0$ as $n \to \infty$

is proved similarly.


  • If $\left \langle {x_n} \right \rangle$ is a sequence in $\R$, the same argument can be used directly.


Also known as

Some sources call this the reciprocal rule, but as that name is used throughout mathematical literature for several different concepts, its use is not recommended.