How can I proof a difficult big o notation? : <p>I can easily understand how to proof a simple big o notation like n<sup>5</sup> + 3n<sup>3</sup> ∈ O(n<sup>5</sup>).</p>

<p>But how can I proof something more complex like 3<sup>n</sup> or 2<sup>n</sup> ∉ O(n<sup>k</sup>)?</p>
