Question

can anyone explain in semi-plain english the concept of Epsilon as used in PSY4 part 4?

Answer 1

in my opinion you should think about epsilon as allowed mistake. because sometimes you cant get exact result, or it would take to much time, or some other reason... you have epsilon.

in part 4 you will have something like:
while something > epsilon
(...you can withdraw each year from your retirement fund, such that at the end of your retirement,
the absolute value of the amount remaining in your retirement fund is less than epsilon)

ideal case would be:
while something != 0
(...you can withdraw each year from your retirement fund, such that at the end of your retirement you don't have any money)

so, using epsilon wont give you ideal results, but it will be precise enough. difference between 0 and 0.001 isn't big.

Answer 2

More simply epsilon is a way of approximating the answer. so rather than comparing two values, you simply compare their difference and check that its less than epsilon(i.e. abs(x^2-a^2)<epsilon), epsilon is then just any small number which is the maximum error we will allow in our result