3x+1 is a conjecture which you can pick any positive integer using the formula: if it is even, divide it by 2, if it is odd, multiply by 3 and add 1. The conjecture states that all positive integers will eventually return to the sequence 4, 2, 1 repeating. This is used to calculate all the numbers in a list for a specific positive integer. It shows the start, end, and maximum.