(1) Arabic numerals, belonging to five elements (categories): 1, mathematics, odd numbers are yang and even numbers are yin. 2, easy to learn, (1) gossip and five elements, water is yang (odd number is water). Fire is yin (even number is fire). (2) The five elements of mathematics, 1, 2, are wood. Three, four, fire. 5, 6, for the soil. 7, 8, for gold medal. 9. 10 is water. Over 10, minus 10. -and so on, too numerous to mention. (2) The above is for reference. I want to know what kind of research you do. Number five?