What is a defining feature of an integer linear program?

An integer linear program is characterized by the inclusion of restrictions on certain variables that require them to take on integer values. This defining feature is crucial because it distinguishes integer programming from linear programming, where variables can take on any real number value, including fractions. The requirement for integers allows integer linear programs to model scenarios where discrete decisions are necessary, such as the number of items to produce or the allocation of resources in whole units.

Integer restrictions can be applied to all variables or just some of them, creating several classifications like pure integer programming, mixed-integer programming, or binary integer programming. The ability to impose these restrictions is essential for addressing complex decision-making problems where continuous solutions are not practical or possible, such as project selection, scheduling, or resource allocation tasks. Thus, the emphasis on including integer variable restrictions is what fundamentally defines an integer linear program.