Negative of Real Function that Decreases Without Bound

Theorem
Let $f: \R \to \R$ be a real function.

Then:


 * $\displaystyle \lim_{x \mathop \to +\infty} f\left({x}\right) = -\infty \implies \lim_{x \mathop \to +\infty} -f\left({x}\right) = +\infty$


 * $\displaystyle \lim_{x \mathop \to -\infty} f\left({x}\right) = -\infty \implies  \lim_{x \mathop \to -\infty} -f\left({x}\right) = +\infty$

Proof
Suppose $\displaystyle \lim_{x \mathop \to +\infty} f\left({x}\right) = -\infty$.

Then by the definition of infinite limits at infinity:


 * $\forall M < 0: \exists N > 0: x > N \implies f\left({x}\right) < M$.

But $M < 0 \iff -M > 0$.

Likewise $f\left({x}\right) < M \iff -f\left({x}\right) > -M$.

Putting $M' = -M$:


 * $\forall M' > 0: \exists N > 0: x > N \implies -f\left({x}\right) > M'$.

The result then follows from the definition of infinite limits at infinity.

The proof for $\displaystyle \lim_{x \mathop \to -\infty} f\left({x}\right) = -\infty$ is analagous.

Also see

 * Negative of Function that Increases Without Bound