666
Cardinal	Six hundred and sixty-six
Ordinal	666th
Factorization
Roman numeral	DCLXVI
Binary	1010011010
Hexadecimal	29A

Naturally enough, 666 also has many mathematical properties, some more interesting than others:

• 666 in binary translates to 1010011010

• 666 is the 667th non-negative integer.

• 666 = 2 × 3² × 37 is the 333rd even number and the 544th composite number. (The 666th composite number is 806 = 2 × 13 × 31). 666 is the 260th non square-free number: {4, 8, 9, 12, 16, 18, 20, 24, ..., 648, 650, 652, 656, 657, 660, 664, 666, ...}. The 666th non square-free number is 1701.

• 666 is not a prime (it is quite evidently divisible by 2, 3 and 111). The 666th prime is 4973. 666 is not a lucky number. The 666th lucky number is 5559.

• 666 has 12 divisors: {1, 2, 3, 6, 9, 18, 37, 74, 111, 222, 333, 666}. The sum of its divisors is 1482. The first numbers in the interval [1, 10⁶] that have the same sum of divisors are 666, 1017, 1481. The sum of its proper divisors is 816>666, so 666 is the 162th abundant number. 666 has 3 distinct prime divisors {2, 3, 37} (&omega(666) = ω(2 × 3² × 37) = ω(2) + ω(3²) + ω(37) = 1+1+1=3) and a total of 4 prime divisors (Ω(666)=...=4).

• By Euler's totient function φ(666) = 216. 666 is the 14th number with this value of φ: {247, 259, 327, 333, 351, 399, 405, 436, 494, 518, 532, 648, 654, 666, ...}

• The harmonic mean of the digits of 666 is an integer: 3/(1/6 + 1/6 + 1/6) = 6. 666 is the 54th number with this property.

• 666 is the 25th repdigit.

• 666 is the 76th palindromic number in base 10 and it is also the sum of two consecutive palindromic primes: 666=313+353. It is the 62th palindromic composite number.

• The sum of all the integers from 1 to 36 inclusive is 666. This means 666 is the 36th triangular number. (The 666th triangular number, on the other hand, (1+2+3+...+664+665+666) is 222111.)

                                   *      
                                  * *      
                                 * * *      
                                * * * *      
                               * * * * *      
                              * * * * * *      
                             * * * * * * *      
                            * * * * * * * *      
                           * * * * * * * * *      
                          * * * * * * * * * *      
                         * * * * * * * * * * *      
                        * * * * * * * * * * * *      
                       * * * * * * * * * * * * *      
                      * * * * * * * * * * * * * *     
                     * * * * * * * * * * * * * * *    
                    * * * * * * * * * * * * * * * *    
                   * * * * * * * * * * * * * * * * *   
                  * * * * * * * * * * * * * * * * * *   
                 * * * * * * * * * * * * * * * * * * *   
                * * * * * * * * * * * * * * * * * * * *   
               * * * * * * * * * * * * * * * * * * * * *   
              * * * * * * * * * * * * * * * * * * * * * *   
             * * * * * * * * * * * * * * * * * * * * * * *   
            * * * * * * * * * * * * * * * * * * * * * * * *   
           * * * * * * * * * * * * * * * * * * * * * * * * *   
          * * * * * * * * * * * * * * * * * * * * * * * * * *   
         * * * * * * * * * * * * * * * * * * * * * * * * * * *   
        * * * * * * * * * * * * * * * * * * * * * * * * * * * *   
       * * * * * * * * * * * * * * * * * * * * * * * * * * * * *   
      * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *   
     * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *   
    * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *   
   * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *   
  * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *  
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

For the year, see 666 AD.