+ 1 are prime, as well as the only prime number 
  
    
      
        p
      
    
    {\displaystyle p}
   such that 
  
    
      
        p
      
    
    {\displaystyle p}
   - 1 yields another prime number, 2. A triangle is made of three sides. It is the smallest non-self-intersecting polygon and the only polygon not to have proper diagonals. When doing quick estimates, 3 is a rough approximation of π, 3.1415..., and a very rough approximation of e, 2.71828...
3 is the first Mersenne prime, as well as the second Mersenne prime exponent and the second double Mersenne prime exponent, for 7 and 127, respectively. 3 is also the first of five known Fermat primes, which include 5, 17, 257, and 65537. It is the second Fibonacci prime (and the second Lucas prime), the second Sophie Germain prime, the third Harshad number in base 10, and the second factorial prime, as it is equal to 2! + 1. 3 is the second and only prime triangular number, and Gauss proved that every integer is the sum of at most 3 triangular numbers. 3 is the number of non-collinear points needed to determine a plane and a circle.
In typefaces with text figures, on the other hand, the glyph usually has the height of a lowercase letter "x" and a descender: "". In some French text-figure typefaces, though, it has an ascender instead of a descender. A common graphic variant of the digit three has a flat top, similar to the letter Ʒ (ezh). This form is sometimes used to prevent falsifying a 3 as an 8. It is found on UPC-A barcodes and standard 52-card decks. Mathematics
3 is the second smallest prime number and the first odd prime number. It is the first unique prime, such that the period length value of 1 of the decimal expansion of its reciprocal, 0.333..., is unique. 3 is a twin prime with 5, and a cousin prime with 7, and the only known number 
  
    
      
        n
      
    
    {\displaystyle n}
   such that 
  
    
      
        n
      
    
    {\displaystyle n}
  ! - 1 and 
  
    
      
        n
      
    
    {\displaystyle n}
  !