Fast Elitist NSGA 3
the parent considers the second objectiveof keeping diversity among obtained solutions.
To maintain diversity, an archive of non-dominated solutions is maintained. The child
is compared with the archive to check if it dominates any member of the archive. If yes,
the child is accepted as the new parent and the dominated solution is eliminated from
the archive. If the child does not dominate any member of the archive, both parent and
child are checked for their nearness with the solutions of the archive. If the child resides
in a least crowded region in the parameter space among the members of the archive, it
is accepted as a parent and a copy of added to the archive. Later, they suggested a multi-
parent PAES with similar principles as above. Authors have calculated the worst case
complexity of PAES for
evaluations as
$()*
, where
(
is the archive length. Since
the archive size is usually chosen proportional to the population size
, the overall
complexity of the algorithm is
&'
.
Rudolph [8] suggested, but did not simulate, a simple elitist multi-objective EA
based on a systematic comparison of individuals from parent and offspring popula-
tions. The non-dominated solutions of the offspring population are compared with that
of parent solutions to form an overall non-dominated set of solutions, which becomes
the parent population of the next iteration. If the size of this set is not greater than the
desired population size, other individuals from the offspring population are included.
With this strategy, he has been able to prove the convergence of this algorithm to the
Pareto-optimal front. Although this is an important achievement in its own right, the al-
gorithm lacks motivation for the second task of maintaining diversity of Pareto-optimal
solutions. An explicit diversity preserving mechanism must be added to make it more
usable in practice. Since the determinism of the first non-dominated front is
&
,
the overall complexity of Rudolph’s algorithm is also
&'
.
3 Elitist Non-dominated Sorting Genetic Algorithm (NSGA-II)
The non-dominated sorting GA (NSGA) proposed by Srinivas and Deb in 1994 has
been applied to various problems [10, 7]. However as mentioned earlier there have been
a number of criticisms of the NSGA. In this section, we modify the NSGA approach in
order to alleviate all the above difficulties. We begin by presenting a number of different
modules that form part of NSGA-II.
3.1 A fast non-dominated sorting approach
In order to sort a population of size
according to the level of non-domination, each
solution must be compared with every other solution in the population to find if it is
dominated. This requires
$%+
comparisons for each solution, where
is the num-
ber of objectives. When this process is continued to find the members of the first non-
dominated class for all population members, the total complexity is
$%&
. At this
stage, all individuals in the first non-dominated front are found. In order to find the
individuals in the next front, the solutions of the first front are temporarily discounted
and the above procedure is repeated. In the worst case, the task of finding of the second
front also requires
&
computations. The procedure is repeated to find the sub-
sequent fronts. As can be seen the worst case (when there exists only one solution in