Step 1: Recall the condition for strict increase.
A function \( f(x) \) is strictly increasing on an interval if its derivative is positive on that interval. That is,
\[
f'(x) > 0 \text{for all } x \in \mathbb{R}.
\]
Step 2: State the condition.
Thus, for \( f(x) \) to be strictly increasing for all \( x \in \mathbb{R} \), the condition is:
\[
f'(x) > 0 \text{for all } x \in \mathbb{R}.
\]
Final Answer: \[ \boxed{f'(x) > 0 \text{ for all } x \in \mathbb{R}} \]
Let \( f: \mathbb{R} \to \mathbb{R} \) \(\text{ be any function defined as }\) \[ f(x) = \begin{cases} x^\alpha \sin \left( \frac{1}{x^\beta} \right) & \text{for } x \neq 0, \\ 0 & \text{for } x = 0, \end{cases} \] where \( \alpha, \beta \in \mathbb{R} \). Which of the following is true? \( \mathbb{R} \) denotes the set of all real numbers.