Properties of Limit at Infinity of Real Function
Theorem
Sum Rule
Let $a \in \R$.
Let $f, g : \hointr a \infty \to \R$ be real functions such that:
- $\ds \lim_{x \mathop \to \infty} \map f x = L_1$
and:
- $\ds \lim_{x \mathop \to \infty} \map g x = L_2$
where $\ds \lim_{x \mathop \to \infty}$ denotes the limit at $+\infty$.
Then:
- $\ds \lim_{x \mathop \to \infty} \paren {\map f x + \map g x}$ exists
with:
- $\ds \lim_{x \mathop \to \infty} \paren {\map f x + \map g x} = L_1 + L_2$
Multiple Rule
Let $a, \alpha \in \R$.
Let $f : \hointr a \infty \to \R$ be a real function such that:
- $\ds \lim_{x \mathop \to \infty} \map f x = L$
where $\ds \lim_{x \mathop \to \infty}$ denotes the limit at $+\infty$.
Then:
- $\ds \lim_{x \mathop \to \infty} \paren {\alpha \map f x}$ exists
with:
- $\ds \lim_{x \mathop \to \infty} \paren {\alpha \map f x} = \alpha L$
Combined Sum Rule
Let $a, \alpha, \beta \in \R$.
Let $f, g : \hointr a \infty \to \R$ be real functions such that:
- $\ds \lim_{x \mathop \to \infty} \map f x = L_1$
and:
- $\ds \lim_{x \mathop \to \infty} \map g x = L_2$
where $\ds \lim_{x \mathop \to \infty}$ denotes the limit at $+\infty$.
Then:
- $\ds \lim_{x \mathop \to \infty} \paren {\alpha \map f x + \beta \map g x} = \alpha L_1 + \beta L_2$
Difference Rule
Let $a \in \R$.
Let $f, g : \hointr a \infty \to \R$ be real functions such that:
- $\ds \lim_{x \mathop \to \infty} \map f x = L_1$
and:
- $\ds \lim_{x \mathop \to \infty} \map g x = L_2$
where $\ds \lim_{x \mathop \to \infty}$ denotes the limit at $+\infty$.
Then:
- $\ds \lim_{x \mathop \to \infty} \paren {\map f x - \map g x} = L_1 - L_2$
Product Rule
Let $a \in \R$.
Let $f, g : \hointr a \infty \to \R$ be real functions such that:
- $\ds \lim_{x \mathop \to \infty} \map f x = L_1$
and:
- $\ds \lim_{x \mathop \to \infty} \map g x = L_2$
where $\ds \lim_{x \mathop \to \infty}$ denotes the limit at $+\infty$.
Then:
- $\ds \lim_{x \mathop \to \infty} \paren {\map f x \map g x} = L_1 L_2$