Many of those optimization problems (and others) will be introduced in the book in
a progressive manner to illustrate the design of search components of metaheuristics.
Those optimization problems are canonical models that can be applied to different reallife
applications. Integer programming models belong in general to the NP-complete
class. Unlike LP models, IP problems are difficult to solve because the feasible region
is not a convex set.
Example 1.10 Still open problems. Some problems have not yet been proved
to be NP-hard. A popular example is the graph isomorphism problem that determines
if two graphs are isomorphic. Whether the problem is in P or NP-complete is an open
question. More examples may be found in Ref. [299].