👉 In mathematics, the term "penfold" is often used to refer to a sequence of numbers that are approximately equal to each other but differ from one another by only a small amount. These numbers are commonly used in financial analysis and quantitative finance to model asset prices or rates of return. The penfold formula involves selecting two consecutive terms in a sequence, which are then used as the starting point for the next term. The formula is as follows: x1 = x2 - (x3 -