What is pi, and how did it originate? Succinctly, pi--which is written as the Greek letter for p, or --is the ratio of the circumference of any circle to the diameter of that circle. Regardless of the circle's size, this ratio will always equal pi. In decimal form, the value of pi is approximately 3.14.
Inverted repeat. An inverted repeat (or IR) is a single stranded sequence of nucleotides followed downstream by its reverse complement. The intervening sequence of nucleotides between the initial sequence and the reverse complement can be any length including zero. When the intervening length is zero, the composite sequence is a palindromic sequence.
A palindrome is a word or phrase which reads the same in both directions. Some simple examples are: RACECAR DEED LEVEL PIP ROTOR CIVIC POP MADAM EYE NUN RADAR TOOT Words like LIVE and STRAW (which read EVIL and WARTS backwards) are not themselves palindromes but the phrases LIVE EVIL and STRAW WARTS are. A palindrome is not necessarily a single word.