Table 3.

The parameters of the SR model used in generating expressions.

Parameters	Value	Parameters	Value
Number of generations	200	Allowed Binary operators	+, *, ^, /
Total number of populations	50	Loss function	Mean absolute error
Population size	20	Constraints	{‘^’:(–1,10)}(a)
Maximum length of expressions (total number of nodes)	30 (WWR), 22 (WOR)	Nested constraints	‘^’:{‘^’:0,’/’:1}, ‘/’:{‘/’:0,’^’:1}(b)
Parsimony (factor control the expression complexity)	0.02	Model_selection	Accuracy

(a) The constraint ‘^’:(− 1, 10) says that power laws can have any complexity in the left argument, but only 10 complexity (nodes) in the right argument.

(b) The nested constraints specify how many times a combination of operators can be nested. The constraint ‘/’:{‘/’:0,‘^’:1} indicates that ‘/’ may never appear within ‘/’, but ‘^’ can be nested once in ‘/’.