# The AIMMS Multistart Algorithm

The multistart algorithm

A multistart algorithm calls an NLP solver from multiple starting points, keeps track of (all) feasible solutions found by the NLP solver, and reports back the best of these as its final solution.

Why use multistart?

A multistart algorithm can improve the reliability of any NLP solver, by calling it with many starting points. A single call to a NLP solver can fail (return a status of infeasible), but multiple calls from the widely spaced starting points provided by a multistart algorithm have a much better chance of success.

Basic techniques

In a pure multistart algorithm many local searches will converge to the same local minimum. Computational effort can be reduced if the minimizations leading to the same local minimum point can be identified and combined at early stages. An improvement is to use cluster analysis techniques to identify regions of points that will lead to the same local minimum.

Algorithm used by AIMMS

AIMMS uses a multistart algorithm that does not use advanced cluster analysis techniques, but instead tries to identify areas of points that will lead to the same local solution. These areas are updated (and become larger) whenever a starting point is found that leads to a local solution that has already been found before. An more detailed description of a multistart algorithm similar to the one used by AIMMS can be found in [KT87].

Definitions

The following terminology is used for the multistart algorithm

• Sample points: a set of points that were randomly sampled.

• Cluster point: a point that defines the center of a cluster, i.e., a cluster is a circle/ball with a cluster point as its center.

• Starting point: a point used as an initial solution (“hotstart”) for solving the NLP.

• Local solution: a solution found by the NLP solver (by using a starting point). A local solution belongs to exactly one cluster point. A local solution can be infeasible.

Two algorithms

The multistart module implements two algorithms, namely a basic algorithm and a dynamic algorithm in which the number of iterations is changed dynamically. The inputs for both algorithms are:

• a GMP associated with an NLP,

• `NumberOfSamplePoints`, and

• `NumberOfSelectedSamplePoints`.

The basic algorithm

The basic algorithm employs the following steps:

1. Set `IterationCount` equal to 1.

2. Generate `NumberOfSamplePoints` sample points from the uniform distribution. Calculate the penalized objective for all sample points and select the best `NumberOfSelectedSamplePoints` sample points.

3. For all sample points (`NumberOfSelectedSamplePoints` in total) do:

• For all clusters, calculate the distance between the sample point and the center of the cluster. If the distance is smaller than the radius of the cluster (i.e., the sample point belongs to the cluster) then delete the sample point.

4. For all (remaining) sample points do:

• Solve the NLP by using the sample point as its starting point to obtain a candidate local solution.

• For all clusters do:

• Calculate the distance between the candidate local solution and the local solution belonging to the cluster.

• If the distance equals 0 (which implies that the candidate local solution is the same as the local solution belonging to the cluster) then update the center and radius of the cluster by using the sample point.

• Else, construct a new cluster by using the mean of the sample point and the candidate local solution as its center with radius equal to half the distance between these two points. Assign the candidate local solution as the local solution belonging to the cluster.

• For all remaining sample points, calculate the distance between the sample point and the center of the updated or the new cluster. If the distance is smaller than the radius of the cluster then delete the sample point.

5. Increment `IterationCount`. If the number of iterations exceeds the `IterationLimit`, then go to step (5). Else go to step (1).

6. Order the local solutions and store the `NumberOfBestSolutions` solutions in the solution repository.

By default, the algorithm uses the initial variable values as the first “sample” point in the first iteration.

The dynamic algorithm: first phase

The dynamic algorithm contains two phases. The first phase is similar to the basic algorithm but with some differences. The dynamic algorithm starts by determining the best sampling box for the creation of the random points (in step 0). For the first sample point, which can be an initial point provided by the user or the first randomly generated point, a method is applied to compute an approximately feasible solution (see [Chi04]) to increase the chance that this first sample point will lead to a feasible solution. Finally, if the dynamic algorithm did not find any feasible solution during the first iterations, and all local solutions found contain large infeasibilities, then a heuristic will be used to update the variable bounds (in step 4).

The dynamic algorithm: second phase

The second phase of the dynamic algorithm is only conducted if no feasible solution was found in the first phase, or if the objective values of the feasible solutions found in the first phase vary. The second phase differs for both situations. If no feasible solution was found in the first phase then the algorithm will continue with steps 1 to 4 until a feasible solution is found or the time limit is hit. In each iteration, the algorithm will now use the method for computing an approximately feasible solution for the first randomly generated point. In the other case, in which the objective values of the feasible solutions found in the first phase vary, the second phase will continue with steps 1 to 4 until enough feasible solutions are found to satisfy a Bayesian estimate for the number of local feasible solutions (or if the time limit is hit).

Using the AIMMS multistart algorithm

The AIMMS multistart algorithm is implemented as a system module, with the name `Multi Start`, that you can add to your project. You can install this module using the Install System Module command in the AIMMS Settings menu. The algorithm outlined above is implemented in the AIMMS language. Some supporting functions that are computationally difficult, or hard to express in the AIMMS language, have been added to the GMP library in support of the AIMMS multistart algorithm.

Calling the multistart algorithm

The main procedure to start the multistart algorithm is the procedure `DoMultiStart`. The only mandotory input is a generated mathematical program obtained by calling the `GMP::Instance::Generate` function of the GMP library discussed in Managing Generated Mathematical Program Instances. Therefore the multistart algorithm can be called by using for example:

```MulStart::DoMultiStart( myGMP );
```

Here `MulStart` is the prefix of the multistart module. The behavior of the multistart algorithm is influenced by several control parameters, which are discussed in Control Parameters That Influence the Multistart Algorithm.

Optional arguments

The procedure `DoMultiStart` contains two optional arguments (with a default value of 0) which can be used to specify the number of sample points and the number of selected sample points (as outlined above). If both arguments are not specified (like in the example of the previous paragraph) or are equal to 0, then the multistart algorithm will use the dynamic algorithm, and otherwise the basic algorithm. For example, if

```MulStart::DoMultiStart( myGMP, 20, 10 );
```

is used then the basic algorithm will be used with 20 sample points and 10 selected sample points. If the dynamic algorithm is used then the multistart algorithm will automatically select values for the number of sample points and the number of selected sample points. It is possible to use the dynamic algorithm and specify the number of sample points and the number of selected sample points yourself by calling the procedure `DoMultiStartDynamic`.

Supporting GMP functions

The GMP library contains the following functions to support the multistart algorithm:

Optionally it is possible to (approximately) project each sample point to the feasible region by using the procedure `GMP::Instance::FindApproximatelyFeasibleSolution`.

Modifying the algorithm

Because the multistart algorithm is written in the AIMMS language, you have complete freedom to modify the algorithm in order to tune it for your nonlinear programs.