where Fa(t) is the forecasting result of the demand for
spare parts and Ra(t) is the real demand at period t, fit is
fitness and I is the total time steps before period t.
4.2.2. Coding
Each activation function solution is defined by a chromosome
whose coding is based on the parallel machine
representation. A chromosome consists of three kinds of
genes: constant gene, function gene, and symbol gene,
which is represented in Fig. 6.
The operation sequence of these genes is from left to
right in a chromosome. As an example, consider the following
chromosome: