Definition:Limit of Sequence

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Definition

Topological Space

Let $T = \struct {S, \tau}$ be a topological space.

Let $A \subseteq S$.

Let $\sequence {x_n}$ be a sequence in $A$.

Let $\sequence {x_n}$ converge to a value $\alpha \in A$.


Then $\alpha$ is known as a limit (point) of $\sequence {x_n}$ (as $n$ tends to infinity).


Metric Space

Let $M = \left({A, d}\right)$ be a metric space or pseudometric space.

Let $\left \langle {x_n} \right \rangle$ be a sequence in $M$.

Let $\left \langle {x_n} \right \rangle$ converge to a value $l \in A$.


Then $l$ is a limit of $\left \langle {x_n} \right \rangle$ as $n$ tends to infinity.


If $M$ is a metric space, this is usually written:

$\displaystyle l = \lim_{n \mathop \to \infty} x_n$


Normed Division Ring

Let $\struct {R, \norm {\, \cdot \,} }$ be a normed division ring.

Let $\sequence {x_n} $ be a sequence in $R$.

Let $\sequence {x_n}$ converge to $x \in R$.

Then $x$ is a limit of $\sequence {x_n}$ as $n$ tends to infinity which is usually written:

$\displaystyle x = \lim_{n \mathop \to \infty} x_n$


Standard Number Fields

As:

the definition of the limit of a sequence in a metric space holds for sequences in the standard number fields $\Q$, $\R$ and $\C$:


Real Numbers

Let $\sequence {x_n}$ be a sequence in $\R$.

Let $\sequence {x_n}$ converge to a value $l \in \R$.


Then $l$ is a limit of $\sequence {x_n}$ as $n$ tends to infinity.


Rational Numbers

Let $\left \langle {x_n} \right \rangle$ be a sequence in $\Q$.

Let $\left \langle {x_n} \right \rangle$ converge to a value $l \in \R$, where $\R$ denotes the set of real numbers.


Then $l$ is a limit of $\left \langle {x_n} \right \rangle$ as $n$ tends to infinity.


Complex Numbers

Let $\sequence {z_n}$ be a sequence in $\C$.

Let $\sequence {z_n}$ converge to a value $l \in \C$.


Then $l$ is a limit of $\sequence {z_n}$ as $n$ tends to infinity.


Also see


Sources