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The AIMMS Outer Approximation (AOA) uses an outer approximation algorithm to solve Mixed-Integer Nonlinear Programming (MINLP) problems. Outer approximation is a well-known basic approach to solve MINLP problems. The underlying algorithm is an interplay between two solvers, namely one for solving mixed-integer linear problems and one for solving nonlinear problems.
AOA uses the system module ‘OuterApproximation’. This module contains a standard outer approximation algorithm that is written in the AIMMS modeling language. The user can customize the individual algorithmic steps in order to achieve such goals as
- obtaining better performance,
- obtaining a better solution,
- storing multiple integer solutions found by the algorithm,
- being able to use different solvers NLP/MIP solvers during the algorithm in order to increase the chance to obtain feasible solutions, or even
- being able to modify the submodels in a problem-specific manner in between algorithm steps.
For a full description of the outer approximation algorithm implemented in AIMMS see 
Chapter 21 in the Language Reference.
GMP library
As of AIMMS 3.6, AOA has been reimplemented by using routines from the GMP library. As a consequence of this reimplementation, the AOA algorithm can now be further customized using all available functionality in the GMP library, e.g. to modify the MIP master problem, or add problem-specific constraints. This GMP version of AOA uses the system module ‘GMPOuterApproximation’.
An example of a customization of AOA is provided in this paper about a modified outer approximation algorithm for distillation column synthesis.
Default AIMMS Solver
AOA is available as the default solver in every AIMMS system for solving MINLP problems. The GMP version of AOA is also available in every AIMMS system.
