In much the same way that complex fractals can be described by
simple formulae, so a
mathematical expression that describes a shoe - print can be produced from an image, making it easy to compare
with others.
As I said though, you can't develop a
simple mathematical formula for this, you can develop a
formula to calculate the sum of money you have from saving a given amount each month or year for a given time
with a given return... but calculating the amount that taken out for a given number of years whilst adding the return and deducting the payout....