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Artificial Intelligence and Soft Computing: Behavioral ... - Arteimi.info

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Definition 15.2: The defining length of a schema, denoted by d(H), is the<br />

difference between the leftmost <strong>and</strong> rightmost specific (i.e., non-don’t care)<br />

string positions.<br />

For example, the schema ?1?001 has a defining length d(H) = 4 - 0 = 4, while<br />

the d(H) of ???1?? is zero.<br />

Definition 15.3: The schemas defined over L-bit strings may be<br />

geometrically interpreted as hyperplanes in an L- dimensional hyperspace<br />

(a binary vector space) with each L-bit string representing one corner point in<br />

an n-dimensional cube.<br />

The Fundamental Theorem of Genetic Algorithms<br />

(Schema theorem)<br />

Let the population size be N, which contains mH (t) samples of schema H at<br />

generation t. Among the selection strategies, the most common is the<br />

proportional selection. In proportional selection, the number of copies of<br />

chromosomes selected for mating is proportional to their respective fitness<br />

values. Thus, following the principles of proportional selection [17], a string i<br />

is selected with probability<br />

n<br />

f i / ∑ fi (15.2)<br />

i=1<br />

where fi is the fitness of string i. Now, the probability that in a single<br />

selection a sample of schema H is chosen is described by<br />

mH (t) n<br />

∑ fi / ∑ fi<br />

i =1 i=1<br />

n<br />

= m H (t) fH / ∑ fi (15.3)<br />

i=1<br />

where f H is the average fitness of the mH (t) samples of H.

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