When I read computational complexity I encounter problems like 3-SAT, set cover, knapsack. In the first two variables are discrete. In knapsack the weights and values are integer and all three problems have a combinatorial structure, what I mean is selecting a finite number from a finite set.
How about problems in continuous domains like finding the minimum or maximum of a function in real numbers? Or solving a differential equation to find a function? Can we prove that these problems too are hard or easy in a computational complexity sense?
Finally how do I know, by looking at a problem, if asking "is the problem NP-hard?" is a valid question?