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Evolution strategies Chapter 4

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ES quick overview Developed: Germany in the 1970’s Early names: I. Rechenberg, H.-P. Schwefel Typically applied to: numerical optimisation Attributed features: fast good optimizer for real-valued optimisation relatively much theory Special: self-adaptation of (mutation) parameters standard

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ES technical summary tableau

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Introductory example Task: minimimise f : Rn  R Algorithm: “two-membered ES” using Vectors from Rn directly as chromosomes Population size 1 Only mutation creating one child Greedy selection

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Introductory example: pseudocde Set t = 0 Create initial point xt =  x1t,…,xnt  REPEAT UNTIL (TERMIN.COND satisfied) DO Draw zi from a normal distr. for all i = 1,…,n yit = xit + zi IF f(xt) < f(yt) THEN xt+1 = xt ELSE xt+1 = yt FI Set t = t+1 OD

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Introductory example: mutation mechanism z values drawn from normal distribution N(,) mean  is set to 0 variation  is called mutation step size  is varied on the fly by the “1/5 success rule”: This rule resets  after every k iterations by  =  / c if ps > 1/5  =  • c if ps < 1/5  =  if ps = 1/5 where ps is the % of successful mutations, 0.8  c  1

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Illustration of normal distribution

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Another historical example: the jet nozzle experiment

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Another historical example: the jet nozzle experiment cont’d

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The famous jet nozzle experiment (movie)

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Genetic operators: mutations (2)

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Representation Chromosomes consist of three parts: Object variables: x1,…,xn Strategy parameters: Mutation step sizes: 1,…,n Rotation angles: 1,…, n Not every component is always present Full size:  x1,…,xn, 1,…,n ,1,…, k  where k = n(n-1)/2 (no. of i,j pairs)

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Mutation Main mechanism: changing value by adding random noise drawn from normal distribution x’i = xi + N(0,) Key idea:  is part of the chromosome  x1,…,xn,    is also mutated into ’ (see later how) Thus: mutation step size  is coevolving with the solution x

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Mutate  first Net mutation effect:  x,     x’, ’  Order is important: first   ’ (see later how) then x  x’ = x + N(0,’) Rationale: new  x’ ,’  is evaluated twice Primary: x’ is good if f(x’) is good Secondary: ’ is good if the x’ it created is good Reversing mutation order this would not work

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Mutation case 1: Uncorrelated mutation with one  Chromosomes:  x1,…,xn,   ’ =  • exp( • N(0,1)) x’i = xi + ’ • N(0,1) Typically the “learning rate”   1/ n½ And we have a boundary rule ’ < 0  ’ = 0

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Mutants with equal likelihood Circle: mutants having the same chance to be created

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Mutation case 2: Uncorrelated mutation with n ’s Chromosomes:  x1,…,xn, 1,…, n  ’i = i • exp(’ • N(0,1) +  • Ni (0,1)) x’i = xi + ’i • Ni (0,1) Two learning rate parmeters: ’ overall learning rate  coordinate wise learning rate   1/(2 n)½ and   1/(2 n½) ½ And i’ < 0  i’ = 0

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Mutants with equal likelihood Ellipse: mutants having the same chance to be created

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Mutation case 3: Correlated mutations Chromosomes:  x1,…,xn, 1,…, n ,1,…, k  where k = n • (n-1)/2 and the covariance matrix C is defined as: cii = i2 cij = 0 if i and j are not correlated cij = ½ • ( i2 - j2 ) • tan(2 ij) if i and j are correlated Note the numbering / indices of the ‘s

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Correlated mutations cont’d The mutation mechanism is then: ’i = i • exp(’ • N(0,1) +  • Ni (0,1)) ’j = j +  • N (0,1) x ’ = x + N(0,C’) x stands for the vector  x1,…,xn  C’ is the covariance matrix C after mutation of the  values   1/(2 n)½ and   1/(2 n½) ½ and   5° i’ < 0  i’ = 0 and | ’j | >   ’j = ’j - 2  sign(’j)

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Mutants with equal likelihood Ellipse: mutants having the same chance to be created

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Recombination Creates one child Acts per variable / position by either Averaging parental values, or Selecting one of the parental values From two or more parents by either: Using two selected parents to make a child Selecting two parents for each position anew

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Names of recombinations

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Parent selection Parents are selected by uniform random distribution whenever an operator needs one/some Thus: ES parent selection is unbiased - every individual has the same probability to be selected Note that in ES “parent” means a population member (in GA’s: a population member selected to undergo variation)

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Survivor selection Applied after creating  children from the  parents by mutation and recombination Deterministically chops off the “bad stuff” Basis of selection is either: The set of children only: (,)-selection The set of parents and children: (+)-selection

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Survivor selection cont’d (+)-selection is an elitist strategy (,)-selection can “forget” Often (,)-selection is preferred for: Better in leaving local optima Better in following moving optima Using the + strategy bad  values can survive in x, too long if their host x is very fit Selective pressure in ES is very high (  7 •  is the common setting)

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Self-adaptation illustrated Given a dynamically changing fitness landscape (optimum location shifted every 200 generations) Self-adaptive ES is able to follow the optimum and adjust the mutation step size after every shift !

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Self-adaptation illustrated cont’d

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Prerequisites for self-adaptation  > 1 to carry different strategies  >  to generate offspring surplus Not “too” strong selection, e.g.,   7 •  (,)-selection to get rid of misadapted ‘s Mixing strategy parameters by (intermediary) recombination on them

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Example application: the cherry brandy experiment Task to create a colour mix yielding a target colour (that of a well known cherry brandy) Ingredients: water + red, yellow, blue dye Representation:  w, r, y ,b  no self-adaptation! Values scaled to give a predefined total volume (30 ml) Mutation: lo / med / hi  values used with equal chance Selection: (1,8) strategy

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Example application: cherry brandy experiment cont’d Fitness: students effectively making the mix and comparing it with target colour Termination criterion: student satisfied with mixed colour Solution is found mostly within 20 generations Accuracy is very good

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Example application: the Ackley function (Bäck et al ’93) The Ackley function (here used with n =30): Evolution strategy: Representation: -30 < xi < 30 (coincidence of 30’s!) 30 step sizes (30,200) selection Termination : after 200000 fitness evaluations Results: average best solution is 7.48 • 10 –8 (very good)