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Genetic Algorithms as a Tool for General Optimization Angel Kuri 2001

Genetic Algorithms as a Tool for General Optimization Angel Kuri 2001. Optimization. Genetic Algorithms are meta-heuristics based on an analogy with evolutionary processes, where three ideas outstand: a) That any optimizable problem is amenable to an (usually digital) encoding process.

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Genetic Algorithms as a Tool for General Optimization Angel Kuri 2001

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  1. Genetic Algorithms as a Tool for General Optimization Angel Kuri 2001

  2. Optimization Genetic Algorithms are meta-heuristics based on an analogy with evolutionary processes, where three ideas outstand: a) That any optimizable problem is amenable to an (usually digital) encoding process. b) That a problem thusly encoded is more easily tackled than its non-encoded couterpart. c) That genetic operators called natural selection, crossover and mutation are enough to fully and efficiently explore the solution landscape.

  3. Optimization(Any problem is encodable) For this to be true we have to restrict the solution landscapes to a finite subset of the original ones. This is not a source of serious conceptual or practical trouble since, in all cases, the computational solution of any given problem is digitally encoded.

  4. Optimization(An encoded problem is more easily tackled) This dictum is not obvious. However, the ample evidence supporting it leaves little room for doubt. More importantly, theoretical arguments seem to leave no doubt: a) Genetic Algorithms always converge to the best solution b) Convergence is usually quite efficient

  5. Optimization(Genetic operators yield efficient performance) Our aim when designing Gas is to make the abovementioned convergence as efficient as possible. Two sources of inefficiency have been clearly determined: a) Deceptive functions b) Spurious correlation

  6. Optimization Before discussing how to overcome the aforementioned limitations we shall analyze a canonical GA (or SGA). Then we will describe a non-standard GA (VGA) which was designed to avoid the pifalls of the SGA.

  7. The Simple Genetic Algorithm Although there are many possible varia-tions, the best understood and more widely treated sort of GA is called “Simple”.

  8. Simple Genetic Algorithm • It considers: • Genetic haploid representation • Proportional selection • 1-point crossover • Mutation with low probability • No elitism • Binary strings • Constant sized population • Bounded number of generations

  9. SGA • The SGA starts from a set of binary strings

  10. Evaluation • Every candidate solution is evaluated

  11. Evaluation • Strings are sorted best to worst

  12. Selection • Every string is asigned a probability proportional to its performance

  13. Selection • Probability of Selection

  14. Crossover • A couple of individuals is randomly chosen; a locus is also randomly chosen

  15. Crossover • Genetic contents are exchanged

  16. Crossover • The process is repeated m/2 times, yielding m descendants. • The idea is that best genes survive giving rise to better fit individuals.

  17. Mutation • Once the new population is set, some genes are mutated. • Mutation is effected on a very low probability basis (typically, from 0.1% to 0.5%).

  18. Population’s Behavior The number of schemata “m” of a given schema (H) at time “t” is given by:

  19. SGA where:

  20. SGA • We now illustrate with a simple example. • Keep in mind that the purpose of the example is to transmit the feeling of how a SGA works. • The point to be stressed is that this Gas solve equally well trivial problems as well as very complex ones. • We illustrate both kinds.

  21. Illustrating the SGA • Problem: What is the largest possible value for the expression ? To make the question meaningful we further impose:

  22. Encoding In this case, the GA’s population consists of a set of binary strings of length 10 which we assume to be encoded in positional weighted binary.

  23. SGA’s Limitations • The SGA, as described, does not converge. In fact, if run indefinitely, it will find and loose the solution an infinite number of times. • Furthermore, in some cases it will get stuck in a local optimum.

  24. Vasconcelos Algorithm To overcome the limitations of a SGA we introduced the so called Vasconcelos GA. It displays: a) Deterministic (i -> n-i+1) coupling b) Annular crossover c) Uniform mutation d) Full elitism

  25. Deterministic Coupling

  26. Annular Crossover

  27. Full Elitism

  28. Application Example • Optimization of a function subject to constraints • Linear functions • Linear constraints • Non-linear functions • Non-linear constraints

  29. Fitness Function where s is the number of constraints which are satisfied and

  30. Numerical Representation

  31. Numerical Example

  32. Sample Run

  33. Sample Run

  34. Sample Run

  35. Sample Run

  36. Sample Run

  37. Sample Run

  38. Non-Numerical ExampleTraveling Salesman Problem

  39. Non-Numerical Example In this type of problem it is needed to repair the solutions, since invalid proposals arise on two accounts: a) Because no two cities may be repeated b) Because not all binary combinations are necessarily utilized. Hence, some individuals are unfeasible.

  40. Repair Algorithm To following individual is unfeasible on two accounts: a) One city is off limits (7) b) One city repeats itself (2)

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