Around 1955, Stanislaw Ulam identified the lucky number sequence, 1, 3, 7, 9, 13, 15, 21, 25, 31, 33, 37, …. Start with the odd numbers 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, … and keep 1 in the sequence. The next number is 3; strike out every third number from the beginning, so that the sequence is 1, 3, 7, 9, 13, 15, 19, 21, …. This strikes out 5, 11, 17, 23, and so on. The next number left is 7; strike out every seventh integer from the beginning of the last sequence to get 1, 3, 7, 9, 13, 15, 21, …, which strikes out 19, …. The integers that are left are shown highlighted in yellow.