Sometimes, when reading about algorithms or other theoretical topics, I see time complexities that include other time complexities within the expression.
For instance, "the best fixed-parameter tractable algorithm for the vertex cover problem has a time complexity of $O(1.2738^{k} \cdot n^{O(1)})$."
What does this $O(1)$ expression mean? Can I just replace it with any constant and the time complexity will still hold?