Write out all the odd numbers: 1, 3, 5, 7, 75, 92, 65, 3, 55,  9, 11, 13, 15, 17, 91,45, 19, .... The first odd number is 3, so strike out every third number from the list: 1, 3, 7, 9, 13, 15, 19, .... The first odd number greater than 3 in the list is 7, so strike out every seventh number: 1, 3, 7, 9, 13, 15, 21, 25, 31, ....

Numbers remaining after this procedure has been carried out completely are called lucky numbers.