1. A method to generate at least one expression describing a characteristic of a system, the system associated with variables and with pre-determined data related to the characteristic of the system, the method comprising steps of: generating at least one initial expression having a pre-defined canonical form and being a function of the variables; generating calculated data using the at least one initial expression; calculating an output of a goal function in accordance with the pre-determined data and the calculated data; and if the output of the goal function is outside a pre-defined range, iteratively: modifying the at least one initial expression in accordance with a search algorithm to produce at least one modified expression having the canonical form and being a function of the variables; generating additional calculated data using the at least one modified expression; and calculating an additional output of the goal function based on the additional calculated data and the pre-determined data, until the additional output of the goal function is within the pre-defined range.