This is true for 3 as well (with 
  
    
      
        n
      
    
    {\displaystyle n}
   = 2), but in this case the smaller factor is 1. If 
  
    
      
        n
      
    
    {\displaystyle n}
   is greater than 2, both 
  
    
      
        n
      
    
    {\displaystyle n}
   − 1 and 
  
    
      
        n
      
    
    {\displaystyle n}
   + 1 are greater than 1 so their product is not prime. A natural number is divisible by three if the sum of its digits in base 10 is divisible by 3. For example, the number 21 is divisible by three (3 times 7) and the sum of its digits is 2 + 1 = 3.
Numeral systems
There is some evidence to suggest that early man may have used counting systems which consisted of "One, Two, Three" and thereafter "Many" to describe counting limits. Early peoples had a word to describe the quantities of one, two, and three but any quantity beyond was simply denoted as "Many". This is most likely based on the prevalence of this phenomenon among people in such disparate regions as the deep Amazon and Borneo jungles, where western civilizations explorers have historical records of their first encounters with these indigenous people. List of basic calculations
Science
The Roman numeral III stands for giant star in the Yerkes spectral classification scheme. Three is the atomic number of lithium. Three is the ASCII code of "End of Text". Three is the number of dimensions that humans can perceive. Humans perceive the universe to have three spatial dimensions, but some theories, such as string theory, suggest there are more. Three is the number of elementary fermion generations according to the Standard Model of particle physics. The triangle, a polygon with three edges and three vertices, is the most stable physical shape. For this reason it is widely utilized in construction, engineering and design. The ability of the human eye to distinguish colors is based upon the varying sensitivity of different cells in the retina to light of different wavelengths. Humans being trichromatic, the retina contains three types of color receptor cells, or cones. There are three primary colors in the additive and subtractive models.