Law of 13579: The law of 1, 3,5,7,9 is 2n+ 1, because 1, 3,5,7,9 are continuous odd numbers. 2n is an even number, and even minus 1 is just singular, so the formula is 2n- 1. Arithmetic progression is a common series. If a series starts from the second term and the difference between each term and its previous term is equal to the same constant, this series is called arithmetic progression.

The ancients divided numbers into even numbers and odd numbers, with one, three, five, seven and nine representing odd numbers and four, six, eight and ten representing even numbers. The ancients regarded odd numbers as auspicious numbers, one representing the biggest, the man with high righteousness and three representing longevity. If a woman comes home three days after marriage and is not allowed to do anything for three times, she will change her mind and reverse the bad things, which means hiding harm and being lucky.