There are a few interesting numbers which are mysterious in nature which students don't learn about until they get to more advanced topics in mathematics. Pi is usually the first of these numbers which students learn about and often along with the formulas for finding the circumference and area of circles, students are often challenged to memorize as many digits of Pi as possible. There are even contests to see who can remember the most digits of Pi. However, there are other interesting numbers which students eventually learn about if they wind up taking increasingly advanced mathematics courses. The "Golden Number" or Phi is one of these numbers. It is derived from a number of naturally occurring ratios observed in nature (such as the proportionality of snail shells) and also can be seen in many works of art. Another mystery number is known as Euler's Number or simply e. It is used in formulas involving logarithms which you might encounter in calculus and trigonometry. Yet another mystery number is i or the square root of negative one. Although it is categorized as an imaginary number it has been found to be useful in a number of ways.