Combination Theorem for Sequences/Quotient Rule
Theorem
Quotient Rule for Real Sequences
Let $\sequence {x_n}$ and $\sequence {y_n}$ be sequences in $\R$.
Let $\sequence {x_n}$ and $\sequence {y_n}$ be convergent to the following limits:
- $\ds \lim_{n \mathop \to \infty} x_n = l$
- $\ds \lim_{n \mathop \to \infty} y_n = m$
Then:
- $\ds \lim_{n \mathop \to \infty} \frac {x_n} {y_n} = \frac l m$
provided that $m \ne 0$.
Quotient Rule for Complex Sequences
Let $\sequence {z_n}$ and $\sequence {w_n}$ be sequences in $\C$.
Let $\sequence {z_n}$ and $\sequence {w_n}$ be convergent to the following limits:
- $\ds \lim_{n \mathop \to \infty} z_n = c$
- $\ds \lim_{n \mathop \to \infty} w_n = d$
Then:
- $\ds \lim_{n \mathop \to \infty} \frac {z_n} {w_n} = \frac c d$
provided that $d \ne 0$.
Also presented as
Some treatments of this subject specifically exclude all sequences where the denominators are zero at any point in their domain.
Thus, for example, this is how it is presented in 1960: Walter Ledermann: Complex Numbers:
- If $z_n \to c$ and $w_m \to d$, then
- ... $\text{(iv)} \ z_n / w_n \to c / d$, where ... $w_n \ne 0$ for all $n$ and $d \ne 0$.
However, it is demonstrated within the proof that past a certain $N \in \R$, which is bound to exist, $w_n$ is guaranteed to be non-zero.
The behaviour of the sequence $S = \sequence {\dfrac {z_n} {w_n} }$ in the limit is not dependent upon the existence or otherwise of its terms for $n < N$.
Thus it is not necessary to state that $w_n \ne 0$ for all $n$, and in fact such a statement would unnecessarily restrict the applicability of the theorem to exclude otherwise well-behaved cases where it is desirable that the theorem does apply.
Hence this restriction is not supported on $\mathsf{Pr} \infty \mathsf{fWiki}$.