This comes from the fact that:

  
    
      
        
          ∑
          
            k
            =
            0
          
          
            ∞
          
        
        
          
            1
            
              n
              
                k
              
            
          
        
        =
        1
        +
        
          
            1
            
              n
              −
              1
            
          
        
        
        
          
            for all
          
        
        
        n
        ∈
        
          R
        
        >
        1.
The successive stages of pronunciation for the Old English twā would thus be /twɑː/, /twɔː/, /twoː/, /twuː/, and finally /tuː/. In mathematics
Two is the smallest prime number, and the only even prime number, and for this reason it is sometimes called "the oddest prime". As the smallest prime number, it is also the smallest non-zero pronic number, and the only pronic prime. The next prime is three, which makes two and three the only two consecutive prime numbers. Two is the first prime number that does not have a proper twin prime with a difference two, while three is the first such prime number to have a twin prime. In consequence, the first pair of twin primes, three and five, encase four in-between, which is the square of two, or 
  
    
      
        
          2
          
            2
          
        
      
    
    {\displaystyle 2^{2}}
  . Two is the first Sophie Germain prime, the first factorial prime, the first Lucas prime, and the first Ramanujan prime.