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© C. FERREIRA, 2002 (Terms of Use) ISBN: 9729589054

Gene Expression Programming: Mathematical Modeling by an Artificial Intelligence

Special search operators

In this section:

The evolution of such complex entities composed of different domains and different alphabets requires a special set of genetic operators. The operators of the basic GEA are easily transposed to neural-net encoding chromosomes, and all of them can be used as long as the boundaries of each domain are maintained and the different alphabets are used within the confines of the corresponding domain. Mutation was extended to all the domains and continues to be the most important genetic operator. IS and RIS transposition were also transposed to GEP-nets and their action is obviously restricted to heads and tails. However, special transposition operators were created that operate within Dw and Dt and are responsible for the circulation of weights and thresholds in the population (see their description in section 5.2.1). Furthermore, special mutation operators – direct mutation of weights and thresholds – were also created in order to introduce modification in the set of available weights and thresholds.

The extension of both recombination and gene transposition to GEP-nets is straightforward as their actions never result in mixed domains or alphabets. However, for them to work properly (i.e., to allow an effective evolution), one must be careful in determining which weights and/or thresholds go to which region after the splitting of the chromosomes, otherwise the system is incapable of evolving efficiently. In the case of gene recombination and gene transposition, to keep track of a gene’s weights and thresholds is not difficult and, in fact, these operators are easily implemented and work very well. But in the case of one-point and two-point recombination, chromosomes are split anywhere and it is impossible to keep track of their weights and thresholds. In fact, if applied straightforwardly, these operators produce such evolutionary monsters that they are of little use in multigenic chromosomes. Therefore, for multigenic systems, a special intragenic two-point recombination was created in order to restrict the recombination to a particular gene (see its description in section 5.2.2).

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